How many countries have ever had female rulers? How many of them had female rulers at the same time?
I just don't think there's been enough to claim that a lack of war between them means anything.
This is not exactly what you're asking for, but I think it's still relevant. There's the Castillian Succession war (1475-1479) between Isabella of Castille and her niece Joanna "la Beltraneja". Although Joanna was the king's daughter, her legitimacy was questioned by some who accused Beltran de la Cueva of being her real father.
It wasn't merely a civil war, since Isabella had also the help of the Crown of Aragon, which was her husband's kingdom, whereas la Beltraneja was supported by the kingdom of Portugal after she married its king Alfonso V. The Portuguese Army took some cities in Castille, but it seems the was didn't had that much support inside the country itself, and the hostilities were intermittent enough to allow the Castillian side to take the upper hand. In the end, of course, Isabella's followers won the war and Joanna's husband's death left her without options. The treaties of Alcaçovas in 1479 forced Joanna to either become a nun or become the bride of the Castillian heir. She took vows in 1480.
Edit: Source (sorry, I forgot): Vicente Ángel Álvarez Palenzuela, La guerra civil castellana y el enfrentamiento con Portugal. (1475-1479), Alicante, 2006.
My first instinct is to say Fredegund and Brunhilda from the Frankish kingdoms. Can anybody more familiar with the period verify whether their rivalry turned into open war?
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