I don't imagine you're going to like this answer, but if it were me, I would try to find one of the dobes a new home. Not so much because of the worry about an oops litter (though congrats for thinking ahead about that possibility), as because two puppies raised together (especially littermates) can lead to all kinds of trouble.
Read up on littermate syndrome. Here's a place to start: https://www.dobermantalk.com/puppy-c...-syndrome.html
I don't have any experience or extra knowledge about dog vasectomies, though. Hopefully someone else will chime in with that.