Mixing them would really bring a riot, since those who liked certain characters would be disappointed, especially since the delay between each game can be long. If I liked Karen, and in the first part I see 2-3 old characters available to romance but not her, I would be very pissed. Instead if all of them are available, or aren't (in second part) then I wouldn't complain since nobody is getting a special treatment, everyone has to wait the same amount of time.
Oh, I am definitely NEVER going to do a game with continuing romances EVER again
