So, after years of neglecting this project i started adding local multiplayer support and noticed many bugs and things that could be improved. 

Two Player Local-coop major features:

• You can play in the same keyboard-
  • Player #1: WASD + L-SHIFT | SPACE
  • Player #2: ARROWS + NUMPAD_4|5|6|8
• Lives are per players (will be shared in future updates).
• Score is shared. Each player kill counts toward the same score.
• 25% harder to get a new life by scoring (25K points)
• You can pause the game by pressing "P" (works only in local coop for now).
• Hit "F10" for fullscreen mode.

This is a list of some of the fixes:

• Added "Restrict joining" directly into the options menu (if this is false, unlimited players can join in a hosted match).
• TAB now shows name + playerID + number of lives
• Each new player will have a unique tank color (up to 15 players).
• Each new player (if more than two) will be assigned an empty tile for spawning.
• If "Friendly Fire" is turned off, players no longer collide.
• Improved performance a little.