I think you’re right - if they’re different classes then I think it would need to be two bookings. I successfully used a normal and a prem + voucher together in one booking as they were all the same class of travel.

However my understanding is that the cardholder whose voucher it is needs to travel on any booking made with that voucher. So I suspect the only way to do this is to hope for more club inventory or all go in WT+.

I think I’ve understood your question which essentially is “does the cardholder have to travel on the booking with their voucher” and the answer to that is yes.