I have to say your concept of cu alter and seiba having their spirit essences collide with one another is so infinitely interesting. i am honestly very intrigued by how it feels. does it place them in one another's shoes? it gave me a lot of feels XD
Something like that. I'm still hashing out the details and stuff, but I hope this gives you a clearer idea of what's going on :)
The Splice, as Waver dubbed it, causes a number of problems and operations are on hold until they could figure out what bug caused it and until Da Vinci can find out a safe way to untangle the spliced two.
As of that moment, there doesn't exist an "Arturia Pendragon" nor a "Cú Chulainn Alter.
There are instead a "Cuturia" and an "Artiecu". With each exhibiting more of their original Spirit Origin's traits than the other's, yet they are blended so well that it won't be exactly correct to call them by their original names.
(For ease, they do it anyway. Artie is still mostly Artie and Cú Alter is still mostly Cú)
Master tried to test this by using a command spell summon for Saber Arturia, and instead got both the spliced pair.
For Arturia and Cú Alter, it feels like they are sharing the same headspace, constantly.
Think of it like sharing a tiny one-person room with someone else, where the only way to get any sort of privacy is to keep to your own corner. But even then, your roommate is still aware of your presence as much as you are of theirs.
It becomes especially tricky with emotions and memories. If Arturia feels sad, Cú Alter is going to feel the same, like an echo. Likewise, if she feels elated, he will experience some semblance of it.
But it works the same way for memories, meaning if Cú remembers the events of Singularity 5, Arturia witnesses his bitterness pretty much firsthand.
Feelings escalate rather quickly between them, especially when they are both agitated. Anger tends to snowball and so does arousal.
Physically, it also works that way.
Diarmuid's squeezing Arturia's hand in the last sketch dump, Cú Alter can feel it, just a little bit.
This especially bothers Cú Alter, because Arturia's always getting handled in some way by her friends.
But you can imagine how incredible it does feel when Cú Alter and Arturia interact with each other.
The splice also switched some of their traits, further making the entire situation disadvantageous. As far as they were able to test, they still have their usual skills, but
Arturia Pendragon is still technically classified as a Saber class, but like Berserkers she is weak to all classes and Foreigners.
Cú Alter is still classified as a Berserker, meaning he deals more damage to all normal classes, but he is weak to Archers.
Coupled with their situation with skills, Arturia basically has no Survivability, while Cú becomes a tank. Something like that.
As far as Noble Phantasms go, they are able to switch to a certain degree, because the items recognize the parts of their original owner residing in each other.
Arturia made the mistake of using Gae Bolg, not knowing about the excruciating feedback it gave Cú Alter, thus she was wholly unprepared for the agony that followed especially since she is unable to use Cú's runes. She hasn't tried Curruid Coinchenn yet.
Cú Alter, if he ever attempted to use Excalibur, does not possess the extreme mana output that Arturia does. He wouldn't be able to support it, and even if he tried, he'd fry his circuits.
Thank you for the ask! :)