Witer said to show up after 10pm, so I wait and precisely went to their door at 11 and it just kept me out. I wait the next day and it still kept saying "the door is locked". Am I doing something wrong here?
hmmm, are you sure you tipped Witer the 210 coins and had fun with Hakan after the tree quest? Try asking him for some fun (you can have sex freely with Hakan if you ask him) and see if Witer's 210 coins dialogue option is there just in case (it shouldn't be there anymore).
Or maybe you loaded at some point and didn't do the quest, you could also load a save and try the quest again just to be sure.
I am also assuming you haven't seen it yet (as it isn't repeatable) and didn't accidentally skip the scene by holding CTRL and pressed "no" when they proposed the threesome.
It really is between 22:00 and 23:55, only requirements being having had sex with both once and the glooms quest itself, nothing else.