Originally Posted by spursdebs

You can’t pool vouchers in 2 different names.
you need to make 2 separate bookings one for you and companion and one in Mums name using her voucher. You need to use vouchers at time of booking you can’t add retrospectively.

Originally Posted by vintagepilot

If you add your mum as a third pax, they will not use her voucher, neither can you book and then retrospectively apply a voucher. You can obviously book for 2 and then book via her account for her but they would be separate bookings.

I would think the only way to get you all on one booking would be to call, if it is even possible. You can be a third party nominee on your mum's account if she is not there to speak to them in person.