As someone who is in a two-singer band, just be aware that if you have someone who is a dedicated singer (aka doesn't play other instruments) it will limit some of your options as far as utilizing both singers. For example, we have a female singer who is our "lead" singer, but our male lead guitarist also has a great voice and they share a lot of the singing duties. However, we run into the problem a lot that some of the songs we write just sound better with a guy singing them, but we can't bench our lead singer for entire songs at a time and have her awkwardly stand off to the side when we perform. We eventually solved the problem by having our female singer pick up the keyboard (she was classically trained on piano, so when our original keyboardist left the band we were able to switch her right over) so that she could be contributing to the song even when she wasn't singing. It sounds like you won't have quite as much of a problem with that since your singers are filling very different roles in the band, but its just a potential issue to be aware of.