You can link up a Game Boy Advance with Ecks vs. Sever and a Game Boy Advance with Ballistic: Ecks vs. Sever 2 and still play all the multi-payer modes. The level combinations between the games are slightly glitched, but it still works.
One shot kills
Enter OOHSTOPIT as a password.
Rapid fire for shotgun
Enter MYBIGUN as a password.
Sever Stinger mission
On the Sever mission Stinger (with the guided missiles), find the door where a man comes out when you shoot a chemical. Stand half by the door and half by the wall and turn extremely slowly. The crossed hand will turn to a normal one, allowing you to open the door as you normally would.