You could go for AI War, though, I think I remember seeing an option where you could share control of units with another player.
And while it's not exactly what you're asking for..
Supreme Commander is always a good one vs AI, and they have a rather good single-player campaign.
I would recommend Forged Alliance over the first and second.
SupCom focuses more on the larger scale of things, battles take hundreds of units to break a well-fortified position, so with a No Rush time (This creates a zone around your Armored Command Unit that you cannot leave) of 10 minutes, you can have a base up and running and your initial expansion squad ready with no worries of rushing. Set your Engineers to Patrol and they will automatically repair your structures and reclaim wreckage for Mass, build shield generators covering a cluster of defenses and you're pretty much set. Resources are infinite so long as you have the structures that produce them.
You have the ability to scroll all the way out until you're looking at the entire map, and you can see the icons of units and buildings, and can actually control the battle from up top, no frantic scrolling around the map or clicking on the minimap to get where you need to be.
However, it's also quite easy to take a T3 Engineer (Or three) or a Support Armored Command Unit, load it into a transport, drop it next to your ally and build near them to supplement their defense, or to add to their assault force.
SupCom is not a "fast" game like Starcraft II can be, it can take a while, especially when you're facing a heavily shielded base. I'm also a turtler, and I prefer to turtle, tech up to T3, build up a large enough force and then steamroll my way through things, while I have artillery to help soften things up.
There's also an option to disable the Fog of War entirely, while it's a bit cheap, it allows you to be able to easily detect threats early, like Experimentals or Artillery Stations being built. I use it all the time because I'm not good at RTS games.