Two underbosses? How does that work? I think Neil was the underboss and Paul was his main capo, sort of... as well as his brother-in-law (and cousin) which gave him a of unique role... but Neil was always the underboss.
Two underbosses theoretically wouldn't really work that well I think especially if they're divided by blue/white collar rackets. There would be rivalry and the white-collar underboss would always bring in more cash.
Then when it comes time for a successor there would definitely be war.. and lets be honest, how many bosses have actually named their successors like Carlo did?