To change "all" weapons to 2 multikills, look for the following in your
../cfg/gungame51/gg_en_config.cfg:
// ============================================================================
// >> MULTIKILL OVERRIDE
// ============================================================================
// Notes:
// * Keep this variable set to 0 unless you want to override the values you
// have set in your weapon order file.
// * This will not override hegrenade and knife, these are always 1.
// Default Value: 0
// The amount of kills a player needs to level up per weapon.
gg_multikill_override 0
If you want different multikill values for different weapons, do what Nightmare-DK posted and set them individually in the weapon order file you use.
Satoon