CJB wrote:
Recently I obtained a Hillingdon Community Services card which doubles
as a Library user's card. And put it into the same card wallet as my
Oyster card - in which I also keep my bank card. Suddently my Oyster
card stopped working on trains and buses, and even the Heathrow
Connect portable validators wouldn't recognise it - to considerable
embarrasment. The culprit was the Hillingdon Community Services card -
which seems to use the same technology as Oyster and was causing a
confict. An irritation 'cos now I have to keep them in separated. CJB.

Having been the holder of a cambridge uni card for a while (never used
oyster but had a few freinds who had both) they could get to lectures
fine with both cards being read, but when they tried to get the tube is
complained about unactive oyster card or somthing,

anyway the answer is to put one card on one side of your wallet, the
other on the other side, and a credit card shaped peice of tinfoil
behind each, touch one side of the wallet for oyster, the other side for
door entry