Something like this:
It takes 2 cards - one each side; "back-to-back"
Would that work?
Or would the reader get confused with 2 cards being presented at the same time? I suspect this would be the case.
They would both be NFC cards, but one a "payment" card, and one an "id" card.
(I don't think this is the intended application of the holder; I think it's just meant for visual inspection)
Thinking of this for an elderly person who finds it difficult to get cards (bus pass, payment card) out of & back into a wallet (due to no-longer-nimble fingers) to present to the reader.