You should keep the card you use to travel separate from other cards and oyster card when touching on the ticket reader, otherwise if more than one card is detected you could be refused entry.
Touching cards together may also risk the wrong card being debited for your travel. Only TfL can correctly charge your journey to the card that you intended to pay with. You can visit its website to make the required changes at https://tfl.gov.uk/contactless (opens in a new window).