In my personal experience, the best DP solution is what RobHammer said. Specifically for us it's worked best when she lies on her back with her ass at the edge of the bed, knees pulled back slightly, while I'm standing. Then she uses the dildo on herself for the vaginal workout while I provide the anal.You get the counter-thrust action RobHammer mentioned, and she gets to control the stimulus to her clit as well as the frequency, duration, depth and speed of the vaginal penetration.
It gets pretty crowded down there, so work up to it slowly! Most times, I don't even move much, I just let her control everything and ejaculate once in a while (it gets so excruciatingly tight and the muscle spasms she has when she orgams just milk it right out).
It's fairly rare for us, but memorable! She's squirted a few times doing this, so I'd give it two thumbs **WAY** up.
Good luck, Guy!

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