I believe you need a LocalTalk adapter. I don't know much about this as I never really tried. I recall way back 10+ years ago actually trying just a serial cable between two Macintoshes. It didn't work. So yeah pretty sure it's not as simple as connecting them directly via a serial cable. Some kind of hub/adapter is needed.

Here's a page on lowendmac that explains how it works:

http://lowendmac.com/2014/appletalk-localtalk-and-phonenet/