Hm, I think he didn't mean shops, I think he meant shared unit control.
You can change this via trigger
Player - Set aspect of alliance
e.g. Player - For Player 1 (Red), turn Shared Unit Control On toward Player 2 (Blue)
(sorry, free translation from the german editor
)
Or you put all players that should share unit control in the same force (scenario/force properties) and mark "Use custom forces" / "Fixed Player settings" and for the force itself you mark every box (allied, shared victory, shared vision, team unit control)
So I tranlated from german editor it might not be the correct spelling, but you should be able to find it.