Well, there are two issues.
1) Copying the user info to the 2nd HD
2) Copying the user's home folders to the 2nd HD (or pointing the users at their home folders on the 1st HD).
For the first item, you need to replicate the NetInfo database... you could do this via an export and an import. This is a 1 or 2 minute operation, once you know what you're doing. I haven't done it personally so I can't tell you the steps. Basically it's going to be 'nidump' from the first HD, and 'niload' from Panther.
For the second item, it's probably easiest of you just create symbolic links on the external HD to the home directories on the internal HD. This keeps them in synch, and it works fine. Basically for user john it would be, in the "users" folder, ln -s john /volumes/internal HD/Users/john
You of course need to blow away the original directory first.
This stuff is pretty straightforward, but it's tough to coach someone through this on a BB.