Data point on using 2 upgrade vouchers at once (Barclays Premier Avios issued):

Tried myself for a few days always getting stuck on ethe screen where you should select both vouchers, only let me select one. Tried via iOS Safari, and desktop Chrome and Edge, same issue. Finally called BAEC, explained the issues and was told that they were aware of this issue in the past but as far as she was concerned, it has been resolved in December. I explained that I was happy to screen record the whole process but she said it was not necessary, insisting though that after helping me she would have to raise an internal ticket about it to justify the waving of the phone booking fee… in any case, the person managed to make the booking without issue, explains the T&C and cooling off period, and even that after the 24hrs cooling off, if in need to make a change, I was better off cancelling and rebooking afresh as I would only lose £2 instead of paying £35 change fee.

Shortly after, wife had buyer regret about the destination, and asked me to book somewhere else, so I cancelled the booking on MMB, Avios and vouchers immediately returned. Tried one more time to make the booking myself, but again was not able to select both vouchers, so I called in. This time, the person on the phone encountered the same issue as me, not being able to select two vouchers and getting the message that insufficient voucher were available to book both legs. The person put me on hold and went to speak to the technical team, returned few minutes later to let me know that the only way around it was to making two return booking, one for each person using one voucher each, and then calling back to link the booking so to ensure I could select the seats for both.. Well, I remonstrated saying that I did not believe this would work (seat selection) but I was reassured it would. So I made two bookings, called in and the third person this time sounding quite knowledgeable, confirmed that the linking of the PNR would not give me right to select seats for my wife, so whilst on the line with her. 8 cancelled again this booking on MMB and once she confirmed from her end the recrediting of Avios/vouchers, she proceeded with no further issues making the booking, noting that the person that had told me the wrong info, must not have been experienced as she was not aware of anyone in the Avios office, not able to use two vouchers for a booking…

so what an adventure.. cannot see what they would not fix it anyway so that customers can become self sufficient