thank you for your response. When I try to do a dummy booking I can select only one voucher at the time. Very annoying, as phoning in is usually a pretty painful and long matter, but I'm afraid it's the only solution then.
You're right, the voucher holder has to be on the reservation so it's impossible to do separate bookings under the same name. Although, the other adult has one voucher as well, so we might be able to use that instead.