Hi
I currently have two males together, but have also had two females together and both sexes alone. No problems with any arrangement apart from the odd 'spat' over food.
Get the biggest cage you possibly can with lots of toys, ropes, tunnels etc. as they get bored very easily.
Also, get them out as often as you can do, because once they are tame and used to you they're great fun. Mine have always come to the front of the cage to see what I'm up to when I get near and will walk onto my hands to come out.
Rats have so much more personality than smaller rodents and are fab pets.. Hope you enjoy yours!