The idea is pretty simple - several Players control their own ACU (exclusively), but beside that all have access to all Units&Buildings of the same Army/Team and share the same economy!
I think i already saw that somewhere, but can't refind that mod (if it does still exist)
So if you could point that out it would be nice 
Else i look into Co-op Campaign mod to create a Standalone version of that feature.
The shared armies mod is basically that, but without the controlling your own ACU part. Everyone in your team can control everything.
What you remembering was in one of the co-op betas where all players had there own ACU but could control everyone's stuff with shared mass income and storage.
Thank you all, sadly i can't use FaF atm as it wants me to link my Steam account for no sane reason - thus i have to search the DVD again 
I think it could be a great gamemode, when everyone has their own ACU (restricted to themselfes, with no control by others!) but shares everything else in the Team - so a higher level of coordination is required but also somewhat easyer!^^
SupCom is Predestinated for such a Gamemode with it's economy, as players don't steal eachother resources but instead everyone in the team always know's what the economy of the team allows and the team can dynamicly decide what to prioritize without preventing eachother from decisions!
And Easyer because 2-4ppl just can do more to build economy&infrastructure and control army's more precisely!
Can't wait to look into the code, does someone Know if that of the Coop Campaign Beta is still available somewhere?
I would call preventing Square Enix from taking down FAF for copyright infringement a pretty sane reason.
Know Python? Want to contribute to FAF? https://forum.faforever.com/post/9801