Again, check the T&C, but in my experience these type of vouchers can only be combined when the serial numbers are sequential. If you get a book of 4 then the serials numbers will be one after the other and so they can be combined. If you get two vouchers at two different times the serial numbers will be off I dont think you can combine them.

I'm almost certain that you wouldnt be able to combine the pounds with the dollar voucher.