So are you working with them in the same space they will be living in permanently? If so, that makes things a lot easier! And less likely to cause problems with the move. How long have they seemed bonded for?
I think I would make the pen a bit bigger, and have them spend time in the larger space for a bit while you get their permanent home ready to go. I would also add in a brand new large litter box. One of my metrics for a strong bond is whether they can share a litter box happily (and generally just be all up in each other’s personal space without issues or tension).
I think I would get ready to move them to their permanent home now, and plan to supervise them there for another 24-48 hrs (but chances are you will know right away if things are fine). Honestly I’ve never had issues with the free-roaming part if when I’ve waited long enough in neutral territory. I also have no will power, so I have moved them to their final condo with the intention of keeping them there for 2 days, and then let them run after 12 hours because they just want to explore so badly!
Since your apartment is new to them, there is even less risk of issues, but if you notice some tension you can always make the space smaller for a bit longer. It’s less likely they will go back to square one, it sometimes just takes a day or so for them to get sorted out in the new space.
.
.
.
The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.