Trying to transfer items between two players by using different StackId's results in a
{ "code": 400, "status": "BadRequest", "error": "InvalidParams", "errorCode": 1000, "errorMessage": "Request cannot specify different stacks for each inventory" }
But what would be the reasoning for this? If I as player 1, split one of my items in two stacks and I want to transfer that new stack to another player, it should be possible.
Also, if you specify "default" for both stacks it will still give you this error.