Can I access a secondary hard drive on another computer via AFP

Yes.

Set up sharing on REM to share the root directory of the second drive.

You can right click and do it from the Finder or go in and manage all shares from the sharing system preference.

Well, I ask for the information (CMD-I) of the secondary drive, and set the "shared" box. Then afp is able to mount the secondary disk. Some difficulty may arise from "symbolic links" between drive (the links you create with unix command ln -s ), and I had to use "hard link" ("create alias" from Finder menu, and probably Unix command "ln ", without option "-s").
Note that I have done all this not on the REM itself, by through the "screen sharing" button that appears in the Finder window of the remote disk ! Fine application that avoided me to run between the two computers !