There is no real equivalent of an 'AUI null modem
cable' (should that
be 'null transceiver cable'?)
If you want to link 2 computer-type ports together you need the
equiavelent of 2 transceivers.
This (or devices like it) is probably the only single-piece two station "null modem
adapter":
https://i.imgur.com/qiMh9Tc.jpg
It's a twin tap, with two AUI connectors. If you drop an intrusive BNC or N tap in
there and terminate it, it'd act like a "null modem" for AUI.
If you want to link 2 transceivers together on the AUI
side then
that's a network bridge. Even a basc one is quite a lot of
electronics.
DEC's full wirespeed bridge was supposedly considered something of an engineering
miracle at the time and occupies 2U of rack space!
Thanks,
Jonathan