This patch copies the system of weapon acquirement found in X5 (another game that has both X and Zero as playable characters): Any robot master that unlocks a weapon upon defeat will grant both characters access to their respective weapons.

The Patch will work automatically, but has no effect on the X and Zero game modes, as there is only a single character in those modes.
Patch should be compatible with other hacks available on ROMhacking.net. Using the current version (1.0) will make the game save possibly incompatible with the vanilla game.