It's possible but it's a pain. Almost easier to just put a router in the car and have both connect to that.
With no router you need to setup what's called an AdHoc network. Basically, on one of the computers you setup a wireless adhoc network (using the wireless utility, it's a real pain with the windows builtin utility) and then you connect to this new wireless network from the other computer. Also, to make it easier you should assign manual IP addresses to each computer, like 192.168.10.100 to one of the computers and 192.168.10.200 to the other (doesn't matter the #).