A Spareparts Joque is my most-recommended harness. Would highly recommend it. Both of your partners should fit into a size A. And it gets tossed into the wash.
I have an article I wrote about choosing your strap-on including differences between jock and g-string types:
(Admin feel free to delete if it's rude to link to another site's blog. I'd normally message it, but the OP lacks an account.)
I'd also recommend Tristan Taormino's Guide to Anal Pleasure for Men DVD. It talks about using a strap-on, giving anal sex to men, as well as videos about purchasing, how to properly wear, and using strap-ons. It was very educational and has some sex scenes to get you in the mood too.
(I know it's for use between the two women, but there's good info on it - especially if you ever want to delve into anal sex - even with each other.)