I tried having a Charlie Card and an Oyster Card in the same Oyster Card holder and it didn't work unless I flipped the holder open with one in each half. Even then, I think I had some problems (it might have been just remembering which half was which), so I "solved" the problem by keeping the Charlie Card in my wallet and the Oyster Card in its holder.

OK, somoene is going to ask why I carry my Oyster card around with me. I'm the type of person who carries my passport around with me. At least that way I know where the Passport and Oyster card are. And it makes urgent business trips easier.

Which reminds me of the time I accidentally scanned my Staples card (key-tag) at the Star/Shaws self checkout line and it set off the unknown item warning which a manager had to clear before I could finish checking out.