https://bushfarms.com/beestwoqueenhive.htm
My first experiment with these was 49 years ago. I was just a beginner and I found the number of bees intimidating. The hive was unbelievably strong. I will preface this with the fact that I've done this and think it is usually easier to just run two one queen hives. The biggest problem for me is that you have a super hive with supers stacked up to the clouds and bees everywhere and to do anything with the queens requires moving and disturbing every box. All those bees can be very intimidating, especially to a beginner. I think to be practical it requires a system that does not require moving any boxes to get to either queen.