I’ve done this before when I wanted to do a “holiday” session. Basically I wanted to run Death House from CoS as a one-shot on Halloween, but only 2 of my players were available on Halloween, and also the whole party was in the middle of an actual adventure in the middle of nowhere in my actual campaign. So what I did is said that during a long rest, those two characters had a shared dreamscape which embodied the experience of going through the Death House adventure together. Side note: Death House becomes extremely scary when you only have 2 PCs, even if they are level 5. If I wanted to be able to do this in a hurry for any time some players couldn’t make a session, I would keep those one-shot compendiums from Kobold Press (“Prepared! [1/2/3]”) handy.