To be fair, CTA does
warn against this sort of thing:
Avoid "card clash"!
Whenever you pay, touch only your desired payment card to the Ventra fare reader: we recommend you remove the card from your wallet.
Ventra can accept payments from many different card/payment types. Taking your card out makes sure that the payment method you intended to use covers every part of your trip.
If you want to continue tapping your wallet instead of removing the desired card, however, a RFID shield as mentioned above is probably your best bet.