In message , at 20:30:47 on
Wed, 25 Jun 2014, Scott remarked:
Can Tfl assure me that I get a red light and not credit the wrong card.

If you keep the other card out of the way, I assume this will be the
situation. If you pay using the wrong card, is it the end of the
world?

If you touch in and out with different cards, you will get two
unresolved journeys (unless they a scheme for registering multiple
credit cards to one account and combining all the day's touches - which
I don't think is what they do).
--
Roland Perry