It's not required to be online for coop. You just have to connect another computer to your Lan ip. If you mean two people in same computer, be patient. I'm hoping to do a revision of this game in the coming future.
Viewing post in Battle City Multiplayer comments
In the next days i will upload a version of this game with local multiplayer with the same keyboard and, hopefully, joystick support. Anyway, this version will be just the same game roughly adapted to make the other tank controllable. There will be bugs and maybe incompatibilities. Im doing this because i need it for a game party i will attend and i thought, for the time beign, it's the best i can do. In the future a new version of the game will come with renewed features. Enjoy!