Hi zoe-ball,
Thanks for your post and welcome to the community!
There is no way to link separate QuickBooks accounts, you would need to run the reports in each individual QuickBooks and then export to Excel and merge the data.
Alternately, you could set the client up with one QuickBooks Plus subscription and use the 'Location tracking' feature to setup the branches as separate locations, and then assign the branch when entering the transaction.
This option would depend on if you make one overall VAT return or individual returns for each branch. You can see more on the location tracking feature on our guide here.
Thanks :)