From my experience with doing double dances with a whole mix of dancers, it's going to turn out much better if you have dancers that are more sensually compatible, even if they don't know each other that well - but it also helps tremendously if they are close/friends.
The dances with girls I barely knew or talked to were pretty awkward or average at best because I didn't know what their personal mileage was, so it's usually just smothering body parts on the custy and hoping they knew when to change/rotate spots so the guy could get a good view or feel of both of them in a timely manner. It was moreso to satisfy the
But then the doubles I did with my closer dancer friends (and oddly, one girl who I barely spoke to) were the best because I already knew before then how raunchy or sexually open they were. It's led to some very erotic girl on girl play along with playing with the custy where we just knew who was groping where and for how long. Everything was always in balance, and I've found myself thinking about those particular doubles even to this day. LOL
As a monger... well. I haven't tried a double yet. It's hard enough for me to find one girl that I THINK will give good dances, but two? Maybe one day I'll take the challenge.