Helps if you both have long thin pencil dicks. The harder the better, at least 2H 
Lot of truth in that.
This can be very hard work with a high risk of disappointment. I'm assuming the SP is experienced in this in which case just let her take the lead. Forget what you see in porn she has to be comfortable and in control with this.
Body shapes and sizes will significantly impact on which positions will work, not to mention suppleness/flexibility of the males. Uncomfortable body positions mean you will tire of it very quickly.
Plenty of lube and she will usually need to have warmed up with one on one anal and is relaxed etc.
Both males will need to be very hard throughout. It can be a very mechanical activity rather than sensual and the fumbling around not to mention embarrassment at struggling to find the hole, can lead to the floppies.
Can be helpful if the shorter cock of the two penetrates first. When you are both in (and once again depending on the physiques of all involved) should you attempt too vigorous movements and 'out of synch' one or both will pop out again and you are back to square one.
If successfully achieved DP can be enjoyable for a willing female, but IMO from the male standpoint is largely a psychological turn on rather than a physical one for the reasons given above.
I should add that with one exception at a party many moons ago (disaster) I have only really done this outside of punting with my OH. She now far prefers to use a dildo to replace the other male and which gives her the same pleasure without the gymnastic hassles. In any case these days I am about as supple as a block of concrete
On the basis that everyone should try everything once go for it. Good Luck .... you may need it.
