There are some things to be aware of if you have two machine running different versions of iPhoto. Each iPhoto library can only be directly used by the same version of iPhoto that created it, so you need to keep this in mind when copying libraries from one machine to another.

For example, let's say you have two machine, one running iPhoto 6 and the other running iPhoto 7. If you copy the iPhoto 6 library to the machine with iPhoto 7 and try to open the library up, iPhoto will prompt you to upgrade the library to version 7. Once iPhoto has upgraded the library, you will then be able to use it with iPhoto 7. However, you will no longer be able to copy that library back to the iPhoto 6 computer and use it there, as iPhoto cannot "downgrade" a library to a previous version.

If you need to transfer photos between libraries on two machines, and do not have the option of running the same version of iPhoto on both machines, you can enabled the "Use XML Info Only" option in iPhoto Library Manger's preferences before doing a merge or album copy. This bypasses the need to open the library being copied from, but has the downside that some album/event information will not be copied. (read more about this option here.)

See also:

Merging libraries from two computers

The "Use XML Info Only" option