Before considering what the wider implications of Assad’s announcement might be, we must be clear about what he means. Sharing power over what?
Over the whole of what used to be Syria? But that no longer exists, nor does the power exist to bring into being something over which a central government might rule with a power that one might divide.
Power over the territory is already divided … Is he proposing a cease-fir -in-place? That is conceivable. But it is not clear who or what could enforce it.
Power over the part of Syria that he still controls? Not likely he means that. But maybe he figures that this is his last chance to get international support for his survival.