The most basic way would be to install Wireless Network Adapters in both PCs and create an "Ad Hoc" wireless network. This is one in which the adapters communicate directly with one another, rather than via a Wireless Access Point as in an "Infrastructure" network (normally with a router). You would then enable Internet Connection Sharing in the PC which connects to the modem.
The more sophisticated method is to install a Wireless Router (and if your broadband is ADSL, a combined Router/ADSL modem as a USB modem cannot be used with a router). This allows either PC to go online without being dependent on a "host" PC running and connected to the internet.