it would be ideal if the
entire iTunes folder (not just the iTunes
music folder) were on the external. launch iTunes while holding the
option key, click on +choose library+ when prompted, and select the iTunes folder on the external.
do this on each Mac once and they both will use the same library henceforth upon launching iTunes. obviously, the external needs to be mounted on either machine's desktop
before you launch iTunes to make it work. also, it goes without saying that only one Mac @ a time can access the library.
J
GG