USB is a master/slave connection, and your intent is to connect
two masters. It can be done, by a cable that supports a
dual-port memory which is a slave to both the masters.
And, some software.

However, a peer-to-peer connection is far easier and can
generalize to more than two computers. Firewire and Ethernet
and variations on WiFi are the best choices here.

Those little USB keychain flash drives are widely available,
inexpensive
and completely supported by modern OS software. Use them if
it has to be a USB solution.