Too much exercise at a young age can affect their joints, this includes running, both on walks and in the garden. I would especially be concerned about hips in a GSD (not sure what hips are like in Northern Innuits!) Did the breeders give you a guide to exercising the pups? How much, what type etc? How much exercise are they getting?

They (behaviourists and trainers) reckon that if you have two pups together, to get the bets ot of them training wise, you need to ensure the pups spend twice as much time alone with you as they spend with each other. Not easy to do, so walking them seperately makes things easier.

I don't doubt dogs get jealous, but they don't get jealous the other can run faster. From what you have said about Cobi so far, (growling with a bone, both at you and Maya, now this) he seems a little insecure, so maybe try and work on building his confidence.