Hmm, might be worth a call in to the gold/platinum support line. I definitely was able to use a companion voucher for my friend and his wife by calling in to the line. They are traveling with my wife and I (I used a voucher for us too) but they’re on a separate itinerary since they can only have one voucher per itinerary. So I would think it would be possible if you booked the trip for them at the very least.