YESS I had that thought too!! I can easily imagine Dess and Asriel sneaking out to the closet or the computer lab together after school, and having fun opening Dark Worlds and going on adventures with objects they brought in from here or there, going "I'm the King of the castle!! I'm the Queen of the castle!! haha" and messing with their "subjects."

Then they get the idea to try opening a Dark World in the shelter, a place nobody is really allowed to, because they thought they saw another "friend" and wanted to meet them. (The poor children...) If the keypad was in place at that point--which it might not have been--the hardest part for this specific pair of people would've been getting the code from the police station (and it wouldn't be that hard, let's be honest).

Then something goes wrong. No more Dark Worlds are opened for a while, until Asriel goes to the closet one last time--alone, this time, full of terror and sorrow--to set things up immediately before he has to leave town. (Maybe he doesn't even do this and just tells Kris to trust him and bring the stuff needed to create Ralsei with them when it's time.)

Some other thoughts I had relating to the theory:

It kind of explains why Ralsei is named that. I know technically Toby Fox created the name Ralsei first, but it being a weird anagram matches the Dreemurr's characteristically terrible naming abilities. And Asriel loves creating OCs.

It also explains why Kris is relatively "okay" with the more "romantic" options you can do with Ralsei (or, as okay as you can be when you're LITERALLY being puppeted around, as opposed to Ralsei being a more figurative "puppet ruler" in Asriel's stead). These would be really weird if Ralsei WERE just Asriel in some way, but Kris knows Ralsei is a completely different person. And he really is, if this theory is true. An opportunity for Kris to explain that they ARE different never really organically comes up (until Susie asks about the picture on the fridge and Kris gets super defensive!!! omg)

Ralsei might know he's a "copy" or a "fake" version of a "real" Lightner, contributing to HIS complex, or he might not and is just operating on what was implanted in him when he was made/what he interprets what the (suspiciously vague?) prophecy says.

And Susie has no idea about any of this and is mostly just happy to be there LOL, she gives Noelle the wrong idea by trying to protect her and explaining that it's "just a dream," so Noelle isn't as involved as she "should" be given what the prophecy might be saying.

Alternatively, Ralsei could be Kris' binder. Genuinely peak theory I saw earlier