That is surprising. If there is more than one male cardinal in my yard, it usually means there's going to be a battle. Cardinals are very territorial. It's always a hoot to see two bright red boy cardinals beating the crap out of each other while their women sit on the feeder, munching away, talking about the weather or whatever cardinals talk about :)
At one point this afternoon, there were at least 11 in plain view, and 7 of them were on the feeder. They do get a little testy with each other at times, but basically they tolerate each other. I have this one tree in the yard that is full of them. They're easy to see with the leaves all gone. there's also a bush that they like and I see several of them flying in and out of it.
It sounds like you know about cardinals. I have a beautiful, but neurotic one in my yard. It attacks the mirrors, windshield and windows of our vehicles...and starts hitting our kitchen window a little past sun up. It is a beautiful bird, BUT my side mirror is badly scratched, and my kitchen window is 'etched' from the repeated hits (at just about the same spot...like it is a 'bullseye.' I have tried putting black paper up on the inside of the window in question, along with hanging a windsock (with long kitelike tail) hoping that the movement would distract the bird...but it still keeps hitting the cars and window. This started back in September...any suggestions?