Sounds like your pit was previously in a bad situation involving humans. But on the other hand your Min Pin has had you all to his/her self. Small dogs like Min Pins get very possessive I know from the Chihuahua that was mine when I lived at home and now is my Dad’s. My parents also had another Chihuahua that got hit by a car about 4yrs ago and he was super possessive of my mom. When he was laying by her no one not even my dad or me could touch her or when we got near her he woudl start growling. I hate little dogs their so damn mean sorry I know you have a Min Pin and those are so cute and proud of themselves when they walk around lol. Ok anyways back to the problem. They BOTH need to learn to share you not just the Pit. Teach them BOTH that when one is in your lap the other must wait. Be very firm about it and eventually they will learn what is going on.