If two European monarchs were conversing face-to-face or writing letters to one another in a language with T-V distinction, would they have used informal pronouns due to shared rank or formal pronouns out of respect for the other's position? Would the monarch of a less powerful state have needed to be deferential to a more powerful monarch?
As a linguist, I can give you the beginnings of an answer.
Firstly, it would differ in public and in private. Royalty had personal relationships too, and use normal pronouns (you, tu, du, Sie, Lei) when they know the other royal well.
You will notice that the most formal forms and titles use metonymies based on concepts (majesty, excellency, highness) -- which are nouns. These are usually prefaced by a form of possessive adjective -- Your Majesty, His Excellency, etc.
This is why most European languages end up using the third person as the most formal, precious, indirect and respectful form of address: Lei, Lui, Sie...
http://mylanguages.org/italian_pronouns.php
In French this is evident in segmentation-based questions:
"Sa Majesté voudrait-elle assister à ce concert?" "His/Her Majesty, would she like to attend this concert?"
But the simplest answer to your question would be that the highest ranking person would receive their title in address from other royals who were not as high in rank. A princess would address the king the same way any minister or courtier would.