Got a new employee id card today - which is also the RFID card that gets us through the door sensors - and a new ecoPass - the pass that gets us on the local buses for free. The latter used to be just a sticker that went on the former, but the buses are moving to RFID as well, so now its a card. Obviously, they conflict with one another; while the ecoPass is in my wallet, the id card won't open doors.
Googled round a bit about RFID interference and shielding, and ended up trying a piece cut out of the back of a Coke can - cut it a couple of mm too big in every direction and fold the edges over to avoid sharp bits. With this between the two cards, one side of my wallet (the id side) now opens doors again. I assume the ecoPass should work from the other side of my wallet, though I haven't tested that yet.