It could also be a rank issue. Your GSD may want to dictate when you get to interact with other dogs and that is a no-no!
Playtime or rank, it's time to set rules. If the minpin defers to the GSD, then anything you do with both dogs, do it with the GSD first. For example, call your GSD to you, pet him, then send him off and call the minpin to you and pet her. If your GSD comes back, sternly correct him and send him away. If he's had some training, now is an EXCELLENT time to practice a down-stay. Put your GSD in a down after petting him (while on leash), then call your minpin. Correct your GSD for breaking the down and replace him. He MUST respect your space and your desires to interact with your pack on your terms. Now this goes both ways- if your minpin starts to get pushy, the same rules must be applied to her. They'll both soon learn that you call the shots and they can't do anything but wait their turn.
It is VERY hard to raise two puppies at the same time. When it comes time to one-on-one playtime, training, and socialization, I do strongly suggest locking up the other dog so the one you are working with has your full attention. You'll have to do a lot of shuffling and it will be a ton of work to make sure both are properly socialized, exercised, and trained. It's usually hard enough to find the time to devote to one pup, now you have twice the work (if not more).