Speaking for myself (obvi), as someone who is considering the Vision Pro but hasn’t pulled the trigger. My understanding is that when pairing with the MBP there can only be one virtual screen due to wireless bandwidth limitations. If either DTTG or DT are Vision Pro apps – even if DTTG is just an “enabled” iPad app – then they could be in their own window and thus not taking up screen real estate in the desktop window.
At least, that’s how I think it would work; if someone actually has one in hand it would be nice to have confirmation.