Calculating Grand Total from two RGs and displaying it outside both of them

Hi All

I was here a while ago getting stuck with calculating totals and with help I was able to fathom it all out. I’m now trying something more ambitious and am royally stuck again…

Here’s a screen shot of a mockup of what I’m trying to do…I created this in PPT as the actual thing is a bit confusing. I want to achieve a Grand Total in the boxes outlined in red underneath.

My app provides a user, in one screen, with Products from Trade and Private sellers. These are displayed in two repeating groups - one shows Trade products and the other Private Seller products.
The user selects the products they want using Ionic Checkboxes which work. Trade Products are added to the “Trade” Repeating Group at the bottom and Private Seller products are added to the “Private” Repeating Group underneath.

I need to calculate a Grand Total in the red box that shows the total of all/any products picked regardless of whether they’re from Trade or Private Sellers.

To achieve this I’ve been using Custom States and Workflows but I haven’t got anywhere useful. If anyone can assist me with this or just point me in a productive direction, I’d really appreciate it.
Thank you.