As you know from the novel, Vito seemed to have anticipated the possibility of a plot against him by Tess and Clem, which was one of the reasons he kept them apart. But, a dual Donship (Tess and Clem ruling the Corleone family after disposing of the Corleones) would have been an impossibility.
Yes, Vito let them live in separate areas, but in a crisis situation nobody would be able to "keep them apart". I didn't mean a dual Donship, but something like Tessio as don, and Clemenza his underboss. Or Clemenza the don, Tessio the consigliere.
