Good point TB. As soon as Michael took over, even while Vito was still alive, he brought his people, Rocco and Neri in to the inner circle, and not Tessio or Clemenza who were older, and who saw Michael as a "kid." A co-donship with Tessio and Clemenza operating as free agents was indeed not workable and Michael would not have allowed it.

I differ a little on the situation with regard to the olive oil business once Michael moved to Nevada.
Clemenza and his successor Frank Pentangeli ran the day to day operations, and I don't think Michael was as involved in them as Vito had been, so technicaly I don't think they were "under-bosses". Michael had bigger fish to fry, and he was content to continue to make money from the Clemenza/Pentangeli family. Further, he maintained ultimate control over them even though he was not a hands on manager. We see this dichotomy in II when Pentangeli has to come to Michael to work out the trouble with the Rosato Brothers. This isn't a matter of some deadbeat being handled in the loan sharking, or some problem with a bookie ... Michael probably didn't even want to know about that stuff once he moved. Dealing with the Rosatos...wanting to kill them.. was something that could start a war, and Frankie knew it. When Michael refused him Frankie was dismissive of Michael implying he didn't know how to handle "street" things any more because he was living the high life in the Sierras drinking his "champagne cocktails." Michael had to remind him that Frankie's family was "still" named Corleone, and that with regard to dealings with outsiders Michael had the final say. Later Michael explained his whole strategy to Frankie vis a vis Roth and why he wanted Frankie to make the deal with the Rosatos.