They can be, if they know the backstory and nearly everything you need them to know. It can't be someone who doesn't have any experience in it. For instance, I can run something that got as big as Allaria by myself, but I also have mbl, and Reonen, and sometimes MDK, who know enough to pick up the slack if I'm away from the computer and can't answer. 

Personally, I'm a crappy Co-Gm. xD. But Co-Gms can be a huge help with keeping everything running smoothly.