Since this is a new thing that has started since you've been fostering the female, I would think that it may be related to her presence. Even though the boys are both neutered, testosterone is still produced by the pituitary gland. It sounds to me like you may have some good, old fashioned male dominance going on.
If the crating/separating is not an option perhaps you could consider the soft muzzles on both boys until you get this figured out. I had to do it with Zoe (my daughter's 45 pound dog) for about 6 l-o-n-g months to keep her from attacking Rhett and Scarlett. Now, they are the best of buddies.
__________________  Don't get your knickers in a knot. Nothing is solved and it just makes you walk funny. |