Certainly not Fredo.
As we learn in the "fishtank" scene, Clemenza says Vito promised Tessio and him that they could "form our own family" after he was gone. I emphasized "our" because it imples a dual Donship. And that would have been a fatal dilemma for Vito: there's never been a dual Donship. Plus, I doubt that either Tess or Clem would have accepted being an underling to the other. They'd either have fought to the death for supremacy, or divided up Corleone territories among them. Either scenario would have been fatal to the Corleone family.
As for Vito's threat ("I'm going to blame some of the people here"): I think it was a threat of muscle but more of withholding police/political support of the drugs business. Another war, coupled with withdrawal of support for drugs, would have decimated the families even if Vito lost.
Vito was nice enough to characterize Fredo by sayiing "Fredo was....well...." and his voice trailed off. Michael said Fredo was "weak and stupid." But no it could not have been Fredo.
As for Dual Dons, there are two possibilities in the fishtank dialogue. No question Clemenza said "Our own Family." But Clem was no grammarian.. he could have meant families. Assuming that is not the case, we do see something like a dual family in GFII, namely the Rosato Brothers. They seem to operate as partners and as equals. I could see a Clemenza-Tessio alliance of two smallish families with well defined territories and areas of operation.
