and they'll consequently complain that it doesn't work.
It can't "not work". The game was designed for drop in / drop out multiplayer. If it works in single player with one person controlling both main characters, it will work in multiplayer with different people controlling them.
There may be extra work involved in quest design to account for players being able to split up and do different things. Enforcing single player only might cut down on much of the dialog and design work, if that is what you were getting at.
In that case, a 'designed for single player' warning should be sufficient.
Of course it can "not work" if a script say, on campaign start, adds new variables to track to a newly created character to say, fix the currently a bit "very meh" character development aspect (morality system, tracking statistics, religion and maybe other stats, etc.) and maybe that script also wants to add persistent variables attached to a character so that export and import scripts can work by utilization of pre-game-setup-hubs.... (not sure what else to call em, basically an admin room where you can modify the campaign before it begins)
Currently I can't say of course how it will work (if at all!), but my fear is that the way the drop in system currently works we can't have campaigns that span over 2 separate FILE instances in-game (like NWN does it) (ie, a MODULE END | Export all char stats/persistent variables -> LOAD NEXT MODULE | Import all char stats, assign variables). While keeping our characters and quest decisions/dialog choices saved and persistent over various campaigns.
Maybe I am worrying over nothing again ;p