Oddly, they didn't just give everyone control of a single house, a thing the OpenRA engine supports. Instead, they split and sometimes duplicate starting units with no other significant changes to the missions.

There are a ton of options to tweak it, though. You can share or duplicate income, share tech, and increase enemy count/veterency/etc.

The tech sharing is super janky. If your ally has a class of building that you do not, your build options behave as if you have that class of building. As an example, if you are playing a Soviet mission and your ally captures a GDI war factory, they get GDI tanks, as expected. You get Soviet tanks, including a Soviet MCV. If you then capture a GDI conyard, you can build GDI buildings and your ally can build Soviet. The moment you build a war factory, you lose the Soviet tank options and have GDI. Sometimes, sharing just doesn't work, however.