How does that work with a debit card and a credit card, both with contactless chip?
Depends how the reader is coded. I've seen some use pre coded logic to choose a card from whatever are in range, others that refuse to process any card if there's multiple valid cards. That "pre coded logic" probably isn't as clever as it sounds, being most likely just taking payment from the first card that it recognises as valid.
Some readers also appear confused when presented with a range of contactless cards for different purposes, others appear to be able to differentiate.