In a couple of my leagues we have 2 co-commishes that are mostly used to decide pass/veto on trades. Then we have 2 backup co-commishes. If 1 co-commish is involved in the trade, pass/veto is decided by the other co-commish and one of the backup co-commishes. If both co-commishes are involved in a trade, pass/veto is decided by both backup co-commishes.
As far as yahoo goes, you can only have 1 commish that has the power to do all the "commish" things on the site. This is the person who set up the league on yahoo. But just because yahoo only allows one person to have the "commish" duties on the site doesn't mean that you can't have co-commishes to decide the major decisions.
I like the idea of co-commishes and in a few of my leagues everyone is basically a commish because they commish other leagues. In these leagues I try to stay as hands off as possible and have the league vote on most if not all issues. We rarely have any problems because every owner knows how everything is supposed to work. It seems like most leagues that run into problems have a bunch of guys who have no clue how leagues are really supposed to work. I think co-commishes are a good idea.