The Problem:
If only a single tax is extracted, you must apply a single rate to it but it doesn't save the extracted amount:
ie) I may have a receipt that has a mixture of expenses that are both Exempt and GST but doesn't have PST.
- ie) purchase at fuel bar where the purchase is $100. of that amount $80 is fuel where GST applicable, the other $20 is grocery items that have NO tax at all.
Therefore if you apply the GST to the entire amount as suggested the GST amount will be $4.76
$3.80 would be the correct amount on the total $80 fuel, and no amount on the food.
You would have then overstated the GST by $0.96

Many of my clients have invoices which need second tax extraction but also have invoices which only have GST or PST (but not both) - I'd like to be able to publish these as well without having to edit in QBO.