Is there a strap-on that will stimulate a woman externally (her clit) or internally, while she's pegging her man? If there are, which one would be the best one to get?
Well, it is usually the dildo, and not the harness so much. I have a Terra Firma harness we purchased years ago from GV. It has an opening in it that is designed for double headed dildos. My husband purchased a Nexus double dildo for me to use with that. There is a smaller end that is designed to go through the opening in the harness into the female that is actually doing the fucking and provide some stimulation. The other end is larger and is designed for the "recipient." In my case, the smaller end never lined up for proper insertion into me. That surprised me as I have never had any problems before that with the "real thing." YMMV as they say. So, that's one example. I think that there are now some dildos that can be used without a harness. I don't have any experience with those. Good luck!
Another issue with the Nexus was that the end intended for the recipient was rather large, and my husband couldn't handle it comfortably. They came out with a smaller version later. We never tried that.