Hamachi will allow you to create a third network in the 5.x.x.x address space that you can both share using a named network of your choosing. The advantage here is that it will ALWAYS work, no matter where each of you is located, whether in the same room, or separated around the world.
If you only intend to do this when sharing the same room, then you could create a WISP (wireless ISP) router by patching a wireless ethernet bridge to the WAN port. Now you have a local network and internet access, just as if you had your own modem.
[remote ap]<-- wireless -->[wireless ethernet bridge](lan)<-- wire -->(wan)[wireless router]<-- wired/wireless -->[computers]
There are also a few commercial routers that support WISP mode (rare, but they do exist). And any dd-wrt/tomato compatible router can be configured as a WISP router as well.