Out of curiosity, do you think spine or sam are the type to get jealous, and what would that even look like?
Bonus points for how the other spine or sam eases whatever jealous feelings the other may have at that current moment
I'm not sure I can answer this in a general way that's not just talking about headcanons, which would be different from person to person. I don't think there is a true canon way to interpret them. I have a very specific way of writing both characters, myself.
My Sam, in his current state*, is lonely but also pretty self-loathing. He can get jealous of others in the general sense of observing their relationships and wishing his life was together like that, but if he got into a relationship only to be cheated on/left, he'd just attribute it to his own failure and stew on it and not blame the other party (much).
He's desperate enough to jump from relationship to relationship, himself, depending on who's willing to show him any compassion at all, looking for the right person... ...and he's got a LOT of choices now, which is kind of a shock to him... To top it off, he really feels he doesn't have much to lose. The world is ending! Might as well figure out who's best to spend that time with, even if that means... multiple people, depending on whoever else is, for some unfathomable reason, also desperate enough to even consider him (in his mind).
*I have a pretty involved backstory for my version of the guy, but I have no idea what would be the best way to go about sharing it, so it's just kind of on the backburner; it does inform a lot of how I write him, though.
Spine, on the other hand, genuinely cares for Sam way too much to be jealous of him seeking out others for comfort on his own terms, because she's extremely self-assured that he'll eventually come around to her terms in due time; she'd rather he be happy and fulfilled until then. If he leaves through an alternate shortcut, she will tell him to "take care." Once he's satisfied he's done all he can with his human life, he can turn around and leave it behind to join her.
As long as he spends some time with her now and then, she's not gonna lash out or anything... and he always comes back. How can he not? They live in the same apartment! She's EXTREMELY patient, too; every real minute is 23 for her, so one hour real time is close to a whole day for her! If he spends 7 hours away from home exploring the complex, that means she was alone for nearly a week!
She's always there, and never judgmental in the least. Hell, getting her to the point of discussing matters of boundaries civilly involves beating on her until she realizes she's in the wrong, and she accepts that! She's very accepting!
And that means a lot to Sam, even if she freaks him out.