Connect Wi-Fi Hotspot Through OSX Internet
This should trivial to do using something like an Airport Express (or Airport Extreme, though I haven't personally done it there). The method I've used involves activating Internet Sharing from the Sharing Preference pane, selecting Wi-Fi for the signal source and Ethernet (or the second Wi-Fi card, if it's set up as a network interface in the Network pref pane) for the check box for "To computers using:".

Sharing Wi-fi -> Ethernet and Ethernet-> Wi-Fi has always worked fine for me using this method, ever since 10.3 or 10.4.