Age and size usually plays into an aggression or dominance scenario. Not sure of either with yours.

May want to look into a video on the web about sexing, and determine if one of yours actually is a female.

Lastly any type of aggression does not have to be immediate, or in a day or two, but at some point in time instinct and nature will kick in. Instinct will always be stronger than a personality trait.

I guess just make sure your favorite is the bigger of the two.