No more than two, I think, since these count as sleeping rooms to house slaves. I wouldn't even mind seeing two communal bed slots removed for one of these double rooms.
What I'm thinking is, if one slave has high lust and sex-crazed/perverted and the other has low lust/lewdness/prude or anything else that causes sexual interactions to become more difficult, then the chances of rape are high but if both are say, perverted or fickle or have high lust, then they'd have a mutual enjoyment.
I don't know about you having to do something about it. I'd treat it as a stress event like sending a slave to Umbra as a fucktoy. They don't actually complain, just gain a ton of stress. Maybe some red text about them complaining to you/headgirl about it.