Essentially, while there are >=2 different cards to a single joint account it would be useful to have the option to extend the choice of approving a payment to other members of the same joint account.
This is mainly for (e.g. grocery) sites that limit placement of a single card payment method, meaning that only one person will get the notification and the option to approve a payment.
As I was reading I thought it was a good idea, but just to play devils advocate;
What if you get a notification âApprove ÂŁ1000 at Appleâ and youâre like âOooh, itâs my birthday coming up! What a treatâ and it isnât really your partner and youâve just approved it for a scammer that has your card details?
Or if you have a controlling partner and they decline anything they think is frivolous
I donât understand what the issue is with this? If youâre placing an order you can approve it yourself in less time than it would theoretically take to extend the notification 
Or if itâs a trust thing then you really shouldnât have a joint account with them.
If partner A has their account details saved on the site but partner B is doing the order I guess
Ohhh I see!
Why wouldnât you just add your own card on there? So going forward youâd have a choice of 2 cards.
Youâd only have to do it the once because it would be saved. No need to ever mess around extending notifications or anything then.
When purchasing online doesnât Monzo have control over the frame which tells you to approve the transaction? Assuming thatâs the case couldât it be as simple as adding a button to that frame - âask on other personâs deviceâ (or similar) for joint accounts.
That said donât see why banks should have to work around retailers insisting you use a specific card.
It could be (not that any task is necessarily simple) but whether there are any legal/regulatory requirements around it would be the other question to ask.
Essentially the bank would be allowing the purchaser to pass liability onto someone else, so there is an increased risk that things that could go wrong.
I suspect this is the case (Regulations)
Joint Account cards have the individual names on them, not a common Joint account name, so notifying another named person with a different card number may have complications.
@RobertGauld, really like this (quote below), but youâre right itâs a bit daft that some sites work this way
@Revels, good points but I imagine it unlikely that either party would blindly accept any payment leaving their cards so this would only really have utility for things that everyone involved is aware of.
@Ordog as in the text, the issue is related to sites that only allow one card to be on their system at a time that both parties want to use.