[Lizard]

A few more specifics:
Assuming computer A is where your OmniFocus database is now, here's how you'd set it up:
1) Mount the USB thumbdrive on computer A.
2) On computer A, open OmniFocus, then the General tab of the Preferences.
3) In the Database section, click the "Move to a new location" button and browse to somewhere on the USB drive.
4) Quit OmniFocus
5) Mount the USB thumbdrive on computer B.
6) On computer B, open OmniFocus, then the General tab of the Preferences.
7) In the Database section, click the "Select a different database" button and go find the file you put on the USB drive in step 3
8) Now just make sure to quit OmniFocus and eject your USB drive every time you go from one computer to the other.