My wife and I have had several MFM Double Penetration threesomes with friends of ours and every one of them turned out well. My advice is to be comfortable with the person that is joining you and your wife/girlfriend. My wife already knew the other guy, so that made the get together really relaxing.
Double penetration doubles the pleasure and can take a woman to a whole new level of pleasure. She likes to be filled up as most women do and having two erect penises deeply penetrating her meets that need.
To accommodate an extra erect penis comfortably, some women need 20-30 minutes of stimulation. If youre used to intercourse with a partner who has a small or medium-sized penis, you may have been able to comfortably have intercourse before you were fully turned on.
If your partners are bigger, you may need to wait until youre very aroused before moving to penetration. Also, anticipating discomfort causes your muscles, including your vaginal muscles, to tense up, making penetration more difficult. If they give you a long, sensual massage before intercourse, it will help to relax your muscles and also turn you on.
It will, however, take a lot of lube and can be awkward finding the right position at first. My wife prefers to straddle one of us ensuring she takes control of how quickly the penis penetrates her and when she is ready allows the other guy to begin anal penetration from behind.
Usually she prefers the guy with the smaller penis to initiate anal sex. If you have pain when his penis is inserted, have him stop, but not pull out. He should just hold his position while you take several slow, deep breaths and relax the anal muscles. Pushing out (as you would for a bowel movement) will help to open those muscles and allow them relax against the penis. The pain should subside though you may still feel some discomfort from having something pushing into you and being stretched open. Try to stay relaxed as he continues to push his penis in further. Any time it hurts, have him stop while you take slow, deep breaths, push out, and relax until the pain subsides.
If the man exercises patience and control, most women will be able to accommodate any size penis using the above techniques. The incredible sensations of fullness for a woman and tightness for the man make anal sex with a big penis worth the extra time and effort.
One thing to bear in mind is that the guys need to comfortable with being that close though as you will most certainly feel the other guys penis through the thin lining between the vagina and the anus, personally I feel that this adds to the excitement.