Funds are shared between you are your co-commander, immediately you might be thinking how this means you'll have more money. Most other times though they proved to be a nuisance than a bonus. Another time they helpfully covered one of my larger vulnerable units from attacks.
The other thing it encouraged me to do was help them build defences, sometimes they'd forget to and their base would be decimated by a few small units. If there is one thing the co-commander helps with is the pacing of the missions and making them go a bit quicker if you are a turtler. The co-commanders were introduced so all missions could be done in co-operative mode, as somebody who plays my entire single player RTS games alone I was curious to see how this works out.
They have bases situated near you or in the equivalent position on the other side of the level. All the single player missions in Red Alert 3 feature a co-commander, which sadly doesn't really help the game.
