Well, because of that I say that, at least, they should be allies. The real problem is the constant wars (I've tried about 3 campaigns with the same result) between Spain and Austria when both are ruled by the same king. That's a nonsense.
About the lenght of the reign, Carlos I of Spain ruled Spain from 1516 to 1556 and, as Karl V, Austria from 1519 to 1556. It's in 1556 when he realizes of the complexity of an European Empire (his dream), he abdicates to his son Phillip (Spain, Flandern, some territories in France and, maybe, Milan (Phillip was the Duke of Milan)) and to Ferdinand (the crown of The Empire and the austrian territories), dividing the Empire in the Colonial Empire (for Spain) and the German Empire (for his brother). It looks to me as an event is needed ;-)
In 1521 Carlos-Karl goes to Spain, true, because of the Comuneros Revolt. But he doesn´t abdicate to Ferdinand until 1556. I hope not to be wrong, it´s my history (I'm Spanish)

. I think Ferdinand was "the man" of Karl in the Empire and head of his own inheritance (Bohemia and Hungary) in the Empire, but he's not Spanish and I don´t know his history as Carlos'.
During those years, most of the gold of America and the armies of Spain were invested in the war against France (for the control of the German Empire) and protestants (German Princes) and, in fact, Spain and Austria were the same political entity (shared armies, economy, diplomacy, etc). Maybe the forced alliance are the best solution. I don´t know. I gess if with a couple of events the issue should be fixed; one for the alliance and another for the correct abdication of Karl (right now, all the Burgundy inheritance of Austria remains, for ever, as Austrian territories).
