Initially you wont be able to, they will have to figure it out between themselves. There may be some fighting, but once you see the shimmey (sp) dance it should get better. It will be like Viper stated, but I think the dominate one will remain female, and the other turns male. For maroons, its the same. Not hard to take care of, but I have heard that they get very mean, which could cause issues in your 20 gal. over time. HTH