You can’t combine payments sources for the same transaction. It’s just too complicated for banks to handle, that’s why nobody does it.
If a refund is issued, that requires splitting back the amount to each card. What If a card gets declined along the way?
If the same transaction of purchase has more than one item, and the payment was split. Then the customer decided to return it only a single item. Then the would be more than one card to return to, thus, cannot automate the process if the customer is needed to make decision.
I’m not a professional in money things, but these are some of the issue I can pull at the top of my head right now.
I imagine if a pro was asked, a lot of serious issues will arise.
I hope they can fix it all in the future and make stuff easier.