V-Flyer

I need to book two different return flights for myself soon UK to US return.

I've one Credit Card upgrade voucher which as a Silver member I can use for up to 150k points. To get the most value from the points, it seems to make sense to book two one ways US to UK as those flights seem to cost many more points than the outbounds. Does anyone know if that's allowed? Or do I need to use it for one of the returns and then just book the other using points without the voucher?

You can use a voucher for a one-way flight as long as both one-way flights are on the same booking. From this page:

The new Flying Club reward voucher can be redeemed on either an upgrade or a companion reward, whichever you prefer. The best bit? You can redeem them on any flight - either a round trip for yourself or two one-way flights on the same booking for yourself and a companion.