Some people will tell you the sex of a group of dogs doesn't matter at all, but I don't think there is a universal answer on that. It depends on the temperaments of each individual. My girl thinks of herself as a queen. Thankfully neither of my boys has aspirations to being anything more than a naive or prince. But my girl's personality has a lot of mischievous pot stirring in it and although all three dogs on their own are each great and various combinations of two of the three are good. All three of them together often ends up being like world war three with most of the warring being between the boys while the girl looks on with sort of an expression of "what me?, I didn't do anything!"
If your two dogs have a nice life with each other I would be asking myself why I want another dog. If I think it is for the benefit of the first two dogs then that is a silly answer usually. If it is because you have one dog who is getting older and you want a puppy now before the old dog gets too old to put up with a puppy or before the old dog passes on then be honest about that being the reason. I wanted to start training my next performance dog before my girl was ready to retire so the next dog would be ready when she does retire. Right now I am showing both of them in different levels of obedience. Fairly soon Lily will move mostly to rally and Javelin will start moving through the levels of obedience. The long term plan is very workable, but right now I am a juggler.