Basically Underfell Papyrus x Blue or Underswap Papyrus x Underfell Sans. Everyone has their own nicknames for everyone except Blue, so your get to pick their nicknames-

1:One of them accidentally teleports to the other's timeline (I have no other ideas rn I'm sorry-)

2:One of the Fell brothers are injured/sick, and the other Swap brother volunteers to help them until they're better

3:Custom!