I want to setup another company on an existing 7400.

Each company needs to be able to intercom the other.
They each need their own operator and for the AA to answer based on what number was called.

Has anybody setup anything like this?

MMC614 put co.A xtns and co. B xtns into 2 different groups. Do the same for the 2 companies' trunk lines.
MMC 304 block co.A xtn group fom accessing or answering co. B trunks and vice versa for co. B xtns.
MMC 603 include the 2 co. trunks in any group whether LCR or not.
The AA can operate on 2 different plans playing different recordings at one go for calls coming from different trunk groups

Only problem I can see there is only one operator group
if they dial 0

I am using a PRI so the trunks are in one group, but using DID I guess I could send them to a group and then use the Forward Trunk menu or is it the Forward station menu at this point?

You can still use dial nine group just separate who uses what lines as tatenda said earlier on post

set up comapany "A" stn group 5000 & company "B" stn group 5001. then voicemail set up forward station menu 5000 goto menu A & forward station menu 5001 goto menu B. just remeber to set "0" to go to correct operator group in each menu if you are going to use "0". 0 goto ext 5000 in comapny "A" & 0 goto ext 5001 for company "B". you might have to create these 5xxx extensions in the voicemail if not already there.

Also remember to change the operator group in the eclass for each extension of company b (that way if someone presses 0 during the mailbox greeting they get transferred to the correct operator group)