Good one. In another thread, I called for using a dildo or two in addition to your own dick so you can fill up all three holes. But, the strap on would definitely help keep some hands free.
Stridge, I don't know about eeverybody else, but I have said from the beginning "whatever floats your boat" and I agree that they should do what pleases them both as a couple. I just shared how I would handle it and why and what I have heard from people who have tried this.
For me, this would be me taking a loss of enjoyment in order to give more joy to the woman ... not mutual enjoyment ... and the amount I'd have to take is not worth it.
If my wife or GF was extremely adamant about wanting to fuck somebody in the ass with a strap on, I'd tell her we'd have to invite another woman into the bedroom for that purpose so we could run a train:
me > GF/wife > 2nd woman.