Been playing this game since 11 11 11 and yesterday saw something I never imagined could happen. Was going through Volskygge with Lydia and Serana following. Got to the part with a narrow passage with a footplate trap that triggers a single swinging blade. I went through carefully and looked back to see Lydia and Serana going through. They triggered the trap. Blade swung left and hit Serana right into the wall, then the blade swung back to the right and hit Lydia through the other wall. They both dissapeared into the walls and never came back! Eventually, I died and reloaded the save to get them back.

If they didn't die as a result of that, they might have come back if you went through a doorway to load the next cell?

How can you have two companions at the same time?

Yeah I would've thought loading a new cell would bring them back if they didn't die?

If you already have Lydia when you enter Dimhollow crypt, you can have them both as followers. Just don't take Serana to see her dad.

Didn't have a chance to find out, the rest of the area had some draugr that really messed me up. Since the character had been relying on two followers for help....without them the draugr overlord pretty much [censored] me.

Aside from the previous response, there are a few mods that tweak followers in various ways and also allow for more than one follower. I usually keep it down to 2 (if I have any at all), but I've had as many as 5...