@Patch, the reason why I made it so only one tax code can be selected on transaction lines is to reduce bookkeeping errors. There is Tax Audit report which can break-down all transactions for the period by tax code. If you could select multiple tax codes on transaction line, it would make it more difficult to spot bookkeeping errors.
@sharpdrivetek I’m impressed how you put in the work to demonstrate the possible implementation with screenshots. The way I’m thinking about approaching this issue is perhaps abandoning Deduct withholding tax checkbox altogether and making tax codes more powerful so they can handle withholding taxes as well. This would make withholding tax functionality available on all transactions (not just invoices) and would make it easier on data-entry. Sure, tax codes would be more complicated to set up correctly but that doesn’t bother me as they will be available for download on website.