Oh sure they can! (In the world of C&C anyway
.)
The most devistating Team combo you can have with a friend is if one of you play as Malcom Ace Granger (the US Airforce General) and the other plays as the Chinese Infantry General.
After Ace has done bombing the crap out of the enemy defences and war factories, the Chinese friend can Paradrop the Mini-Gunners for a quick capture.
Easy as Pie.