In Tandems the larger, stronger, heavier, more experienced person usually goes in front. I think the issue you are up against is a Western culture thing.
Like men driving in most any man/woman automobile situation. Women drive too. Women can even drive men around & nobody bats an eye or even takes notice. But, for reasons that can't be explained, despite mutual equality, when a man & a woman get in a car together, men end up in the drivers seat greater than 50% of the time. ( For the haters: I'm not a chauvenist, I'm not even sure how to spell it. I harbor no feelings either way. It's just a pragmatic observation of cultural norms.) The there is no female equivalent for the acronym MAMIL, either...Strange?
Why is Base2 writing this? Because on a tandem the larger, stronger, heavier, & usually more experienced rider is usually the man, and for reasons of physics the weight should be up front where the steering happens. It's become convention and tandems are built the way they are based on real reasons of functionality.
Are there exceptions? Sure. Custom built tandems come in all varieties. This option sounds like it may be out of your budget though.
The best tandem you can buy is the one that makes you feel ok with your girlfriend as Captain & you as a helpful stoker supporting wherever she decides you both should go.
Hint: The solution to your inquiry is not about finding the right bike.