Technically you have to book your voucher, and your wife hers. So this will require two separate bookings.

If you are an authorised user for her account you should be able to do these simultaneously on the phone (but would have to pay the offline fees).

Online you have to be logged in as yourself to make your booking, then as her for the second ... or use two browsers.

All the avios come from across your household account regardless of course.