So you have 1 pc that connects to broadband through a USB Modem.
you would need to setup Injernet connection sharing on the system that is connected through the modem.
Then configure both USB wireless adaptors to work in "AD-Hoc" mode.
The second machine will only connect to the internet if the first is on.
A wireless router will enable the internet connection to be independent of either system andwould be permanently available.