This is a hard question to answer with any specifics. Activity can vary widely from one man sitting watching while you play with the other, you giving head to one while the other takes you from behind, right up to double vaginal penetration.
Other than in Option A, there will inevitably be some incidental contact. Really no different than brushing past another man in a crowd. Well maybe a little different.
It comes down to - contact depends on what you do - what you do will depend on what you are comfortable doing.
If your husband doesn't want "balls hanging in his " it just means he won't be eating you out while the other man takes you from behind. Whatever he is comfortable with!
Keep discussing thing, figure out your comfort levels and go with that. Start off slow and move forward from there.
In all likelihood he will be so involved with everything that is going on that he doesn't even notice whether theres been any contact or not. I know I don't.
Jesse