I have something bordering on theory and headcanon that during the sequence of Golem related quests in Act II, Ariel uses thaumaturgy at least three times on screen (besides the ritual itself), and - on top of that - on no other than Wiktor. The case is we can't see it because the narrative is still told from (as the game repeatedly proves, often very unreliable) Wiktor's perspective. Now, bear with me for a few more assumptions I'm making:
• Firstly, I assume that thaumaturges can use the same tricks on each other, or on themselves, as they do on people outside the circle and without any thaumaturgy related abilities. And while I assume this has its own rules, difficulties, and consequences, I see no reason to consider it impossible. In the game itself, we primarily see interactions between thaumaturges based on the use of their abilities, primarily within the context of getting inside each other's heads. White Roman aside, we never experience metaphysical face-slapping between two thaumaturges (Ariel simply chooses physical violence). BUT I assume such an option exists in the universe, simply because we're familiar with the mechanics of fighting other salutators. Finally, following this line of thought, I assume that manipulation between thaumaturges is not out of reach. Perhaps it could be considered more dangerous in a way, and simultaneously facilitated by the awareness of the developing Flaw and its direct connection to the bonded Salutator.
• Something I didn't want to link to the point above, but thematically still remains as an assumption, is the topic of traces left on objects that thaumaturges can read. I wanted to highlight this skill because I'll return to it later, but let's go through it step by step: I don't think it's easy for a thaumaturge to conceal their intentions and feelings so as to mask their traces from anyone who could potentially read them, if that's even possible. Several times in the game, in very different situations and places, Wiktor manages to read very clear traces left by other thaumaturges. While I believe a thaumaturge might not feel the need to cover their own tracks in their own home (Ariel's attic, Emir's house, Stanisław's office, Sofer's office), a thaumaturge who knows someone might be observing them should be conscious of their own mental trace. Stanisław must have realized better than anyone how close the Okhrana were and that they were interested in recruiting a thaumaturge into their ranks; I can't imagine that with such knowledge, he wouldn't deliberately want to minimize the trace of his thoughts and feelings - for example in Svetlana's presence. Again, I'm not saying caution is impossible, I'm just saying that's very human of them.
• To put my line of thoughts in order, thaumaturges obviously influence each other, and while they aren't defenseless against their own kind, I believe their advantage over "ordinary mortals" lies in their knowledge and recognition of their own tricks, not in any particular resistance.
• Another thing I'd like to note is that, as far as the protagonist is able to go with the narrative imperative, I don't consider Wiktor some kind of prodigy when it comes to thaumaturgy. I see him as a gifted thaumaturge with incredible potential, keen in detective work and observation, but also somewhat bullshitting his way through it. Without Rasputin's help, he couldn't grasp the power of more than one salutor (which isn't anything embarassing; Ariel clearly had a major problem with Golem, which could have ended tragically if not for the Coterie. We never received confirmation that Stanisław was capable of it either, despite Wiktor's theorizing). We don't see him inviting himself to other thaumaturges' heads; he's surprised by this trick himself. What I want to emphasize here isn't that Wiktor is weak, because that's not true, but that he's not beyond the reach of others' influence. My point is, /Wiktor is not above it/, but he is incredibly brazen in his, well... pride.
FINALLY returning to Ariel and what we don't see: I believe Ariel used the fact that he's also a thaumaturge against Wiktor three times in a very subtle way, and at least two of them were a simple spite thing, but they also fit perfectly with the main theme of the Golem side plot: revenge. Could Wiktor's perspective not accept this because he simply missed it, or can I write it on blindness of his own flaw? To me it depends on a mood.
A small disclaimer: I know all scenes described here in Polish, so my translation may not be very accurate with the official version, but I hope you'll also remember them somewhat in the language you played the game in.
Case 1: In the conversation scene between Ariel and Wiktor on Bazar Różycki, Wiktor has no option but to admit that he has a sister, even if we choose not to confess about Ligia. Wiktor will say he's alone, Ariel will mutter something half-convinced, and Wiktor suddenly changes his mind and confesses to his twin, which Ariel comments on as a "sudden twist." My theory is that what we don't see is Ariel already having some offscreen information about Wiktor gathered, and at that very moment, something clicked. A notification appeared about a thread Ariel could pull and push Wiktor to tell the truth. We can't deny Ligia's existence because Ariel already knows Wiktor's bullshiting him and he's making him speak one way or another. At this point, Ariel sees a scenario that has played out dozens of times for Wiktor, but this time, he's not the one staging it.
Case 2: Of course, we might never reveal to Ariel that we know Javier is alive (or even simply not find out yet), but for the sake of this theory, I'll assume that this is the default way to proceed with this conversation. Ariel isn't thrilled that Wiktor is reading his correspondence, and while he can't return the favor, a very ironic twist comes to the rescue: for 15 years, the twins have known each other only through letters, and for Wiktor, the thought of Ligia is primarily a thought of an emotionally charged piece of paper. The game emphasizes the sad emotional toll this entails and the burden of rebuilding the relationship between siblings, and I think it's worth remembering this in this quest, which is already so important for Wiktor and Ligia's bond (those who fucked it up big time know what I'm talking about). The twins appear together in the basement of the synagogue and Ariel greets Ligia with the words "dear sister," which sounds very much like something a letter would begin with. "Dear sister" was supposed to remind Wiktor why he should respect people's privacy more; it was supposed to make him a little uncomfortable, and honestly? Since I realized that, this entire sequence has become even more impactful for me.
Case 3: It occurred to me (half-jokingly) that Ariel, besides being truly kind-hearted, once he dropped Ligia (and Wiktor I guess) off at the their home, couldn't resist "sniffing" Wiktor's belongings, just as he had first sniffed Javier's barge and then his attic. I don't assume he had an ulterior motive from the very beginning but rather that he seized the opportunity out of impulse and a reluctance to stay in a worse position, simply because Szulski had touched a napkin that held a bit too much of his sadness.
To finish this incoherent rambling: Ariel Rofe is a reserved, emotional, and very vindictive man. He has a soft hear and although his actions often contradict his harsh words, his Scar has a solid and visible impact on him and all in all I see him as a very tragic character. At the same time, I'm glad the developers decided to allow him to be all "nasty" and microaggressive. Even if, at the end of the day, I'm a little delusional in what I think I see behind his actions, I really appreciate how delving into these understatements—both in his case and in many other characters—allows us to see the multifaceted nature of the side characters as people with their own agendas and flaws and not just as background noise to Wiktor's life. NPCs in The Thaumaturge are fantastic and Ariel is one of my favorite characters in the game and being delusional about him is incredibly entertaining. Thank you for bearing with me for that rant and I'm terribly sorry for my very ass English.