Generally, they don’t end well.
The good people of Andorra and both of its Co-Princes take offense at your gross stereotyping, sir.
Their data point is noted, as is San Marino and Swaziland.
On the other hand, so are Sparta, Rome, Carthage, and all those legendary Scandinavians who kept killing their brothers.
How were these supposed to actually function. Was it just they had to agree to do anything? Or did they have different roles they did?
Mix of both. Sometimes you have rulership switching off between the two on some regular basis, sometimes you have a sacred/temporal division, sometimes you have both with veto power over the other.