I tried the Co-GM thing once with a long running Star Wars campaign. Truth be told, it was an unmitigated disaster. The campaign had been run by one guy who wrapped up the campaign with us surviving a narrow escape from the second death star. My friend and I tried to take up the mantel and keep the campaign going in the New Republic period and despite both loving the setting, we ended up having radically different ideas on how to run things. In addition, we had grown EXTREMELY attached to our PCs after an 18 month campaign and things just never worked well. My situation was obviously unique, but the Co-GMing went so bad, it ended up splitting our group and very hard feelings all around.
"For to win 100 victories in 100 battles is not the acme of skill. To subdue the enemy without fighting is the acme of skill." Sun Tzu - The Art of War