Hi Uwe,
Weight limit has no impact on this. If i understood correctly system is generating transfer order for material but if the stock in two different bin then it is creating two different transfer order in another word it is taking Material + Source Bin into consideration while creating the transfer order whereas your requirement is to create the transfer order based on material only.
Also could you please let me know which document are you using to create this Transfer order which will help me to simulate the scenario and provide the correct solution to you