For some reason if a particular barcode is sold by different salesman to the same customer in a same bill and if salesman code is necessary before saving a bill then the second time we scan the bar code, it does not ask for the salesman code instead of that it automatically increases the quantity in the previous barcode scanned and the first salesman enjoys the incentive.
Ideally in one POS Bill and same Item there is only one Salesman in general Retail operations. As your case is special, you are requested to enable Qty popup option in such Item Master thus it will create two / multiple line in the POS Bill against mutiple scan and will allow you to input multiple Salesperson against them. You are requested to log a Support Ticket against this and get your query resolved.