Hi Mohammed,
I didn't understand on your point " more than one service product to repair order is forbidden"
Why you need to add more than one service product ? Is your question relates to Service Product determination at repair order? which means each time you are getting same service product ?
You also have option to enter the service product manually while creating repair order, here you can change as per your requirement. If you don't want to enter manually but system to determine automatically, then you have to create your own table for VB12 condition records
and as alternative you can create your each branch as a separate work centre and assign respective cost center, when ever you are entering IW41, actual value will be picked up from KP26 with respective cost centre/activity type
you can think of either of these two options to accomplish your requirement
please revert if you still need help
regards,
santosh