Then, I came across this:
Is was glass, which is now my favorite material for back there. Plus, as you can plainly see, it is 2 headed. It was inexpensive ($30) and very well could allow for us to simultaneously enjoy being penetrated even though it is not attached to a strap on harness. I asked her if I could buy this one and she said ........... YES!
It's still bigger than the rubber cock that she currently uses in her harness and is a little bigger than the widest part of my butt plug, but I've got a great deal of confidence.
I'll keep you posted!