Okay, I was writing an answer, it got too long and now I have to head to bed. I'll finish it tomorrow, along with a reply to
As far as I know, co-rulership – official, legal co-rulership, that is – was very rare. Rome provides the most examples by far and, come to think of it, had a fairly good track record.
Few people know that the Roman Republic, save for the occasional dictator, had shared rule, usually 2 for 1 year terms, with military power alternating between the 2 every month. Of course, one might in practice have more power than the other, but for the most part, it worked well since the republic lasted a fair 500 years.
The thing is, I can't judge if this relative stability of the system was because a) checks and balances successfully prevented concentration of power (the Senate mostly, also various powers were delegated to other positions), b) Rome had a strong "civil society", if such a term applies then, that would keep the system going on regardless of ruler mismanagement (a parallel it shares with the USA) or c) because of pure republican spirit e.g. Cincinnatus. We talk about materialism and spooks and all but this republican spirit can't be underestimated, seeing as somehow it took 500 years for one of the dictators to finally coup the system. Consulship continued to exist in the Empire but was mostly a bureaucratic and symbolic role.
Of course, this Roman stability wasn't stable at all by modern standards. Risk of invasions and political intrigue were commonplace, and famously, Rome was at an almost continuous state of external war. But curiously, actual civil wars were relegated to the fifth, last century of the Republic, during which they were constant. I don't know enough of roman history to tell you the details, but I'm guessing that people must have by now studied this sudden shift to look for valuable political lessons.
So yeah, I guess co-rulership is viable, but whether it can be applied universally or Rome was a special case is up for debate.
I'll further discuss this tomorrow.