When we CASSed to Barclays they asked for two cards but I’m not actually sure if card details are a requirement for a switch so it might go through without
Cards aren’t really a requirement, as long as the other details match. I’ve switched accounts without proving a debit card number multiple times and it’s never been an issue
I think some forms made me tick a box stating I didn’t have a debit card. Depends on the bank you are switching to. And whether the switch is accepted depends on the bank you are switching from, afaik.
Might just want to try it - worst case it gets rejected and you try again a few days later
I can’t say for certain, but I would imagine the card number given is checked against active cards on the account.
Therefore, as long as the form allows you to submit only one card number, it should work. If it forces you to type a card number for each account holder, perhaps the same number twice would work?