As long as each male has sufficient room to get away from the divider and hiding places on their side, they should be perfectly fine. Flaring is a healthy exercise for them in moderation. Once they have gotten used to the idea that there is another male beside them, they will simmer down. Give them some time. Blocking the divider with plants is a great way to calm them down a bit although if they really wanted to go over there and flare at the neighbor, plants are not going to stop them. LOL What size tank did you divide? I have a 10 gallon divided 3 ways for my males. Works out great and everyone is nice and happy.