Aw, Bourbon, I absolutely love answering these types of asks, actually. What-if scenarios are so much fun to think about and explore. In fact…if I had a better handle on Cash’s character, I’d probably try to drabble this. As it is, I’m afraid my answer might be inadequate. I’m going to give it my best shot, though!
A huge aspect of Twist’s character is that he does not obey many societal rules. (In the Tale-verses, this might be considered a charming quirk, but in any of the Fell-verses, this is off-putting and even terrifying.) He’d likely invade Cash’s personal space, studying him a lot more closely than is probably appropriate. All the while, he’d be talking, asking questions, sometimes questions that don’t even seem to make sense for the situation. What he’s trying to do is provoke a response, so that he can properly gauge the other monster. Twist is crazy…but there’s some method in his madness.
Now, Cash…I don’t know Cash that well. I’d probably say that his main characteristic is being cunning and calculating. So…I think he’d be evaluating Twist the entire time that Twist is appraising him. He’d likely think that this other Papyrus was cute, in the “what a cute puppy” sense. His first instinct might be to dismiss Twist, and I think he’d try to use some coin trickery to influence Twist, figuring he might make an entertaining toy.
But…I honestly don’t think Twist would be susceptible to that particular trick. He’s just off enough, his logic is just (heh) twisted enough, that he’d be pretty difficult to influence. This might make him a lot more interesting to Cash. (And, of course, once Cash brought out his coin, Twist would probably get excited and want to exchange coin tricks. Because he’s Twist.)
In short, I think these two would spend a good deal of time appraising each other, each in their own fashion. Looking for weak points. Evaluating what kind of monster the other is. (Twist, at least, would be concerned about Blackberry’s safety; he wouldn’t want to introduce his brother to any monster that he considered untrustworthy.) I could definitely see their initial friendship as a means of keeping an eye on the other monster. There’d definitely be some lasting tension between them. As for how they would end up resolving it…?
Shrugs. I think answering that would require a fic. (And for me to get my Cash headcanons together.)