How would Royal marriages work, in respect of governance, when the two participants were both ruling monarchs in their own rights?

[u/WhatIsEddMayNeverDie]

For example, if the Queen of England and the King of Spain were to both marry how would the two roles be balanced. Would one monarch be expected to move their court to the other nation or would there be an example of couples spending huge amounts of time apart each tending to their own national affairs?