I used to do this in NYC in the '80s, but never once had a need to use my second wallet. As noted above the issue is not really mugging (which by definition is someone forcibly taking your wallet by threat or assault) as opposed to pickpocketing, which is generally non-violent theft. You can fight the latter by remaining vigilent and placing valuables in harder-to-access locations. I suppose you could have some fun by carrying a wallet with expired/cancelled credit cards and maybe some worthless Venezuelan bolivars, but really is it worth the trouble?