It would be nice to have a spare in case one runs out of gas..
but seriously..I think everyone has to be comfortable with each other for it to work and that's hard to find in the hobby. Most guys are wanting to be discreet and not meet another man. In the realworld, all you have to do is say, "hey bring your buddy too" .
The younger ones seem to have no problem with it.
Ive always been a one on one kinda girl but I just did my first FFFM (my last review) in the hobby, it was a lot of fun