I had a similar issue with Simon and Magoo. Simon got a long great with ALF when he came to live with me. Enter Magoo about 6 months later...... things were great for about a month and then they started attacking one another, very scary. THey are fine as long as I am not petting or sitting with one in front of the other. I have to split my time between the two of them when it comes to cuddles. I can sit with the Basset Hound, ALF and love on her and neither one cares about that??? Have you tried not being in the room when the trainer is there? Maybe it's a jealousy issue with you like my two.
Simon hates any smushed nose breeds. He had a Smooch the Pooch booth at a HUmane Society event last weekend and was fine with all the other dogs, but when he encountered a Pug, American Bulldog, Bulldog and Boxer her went into attack mode. My other little Bulldog was like that too. She was fine with all other breeds but hated smushed nose breeds.
Good luck and thank you for taking in these two girls and trying very hard to make this work. A lot of people would just hang up the towel and hand them back over to the rescue.