Play as Master and Uranus
While the options are flying on to the Title screen, press
this combination VERY QUICKLY for Player 1: R1, L2, WK, L1,
R2, SK. You should hear a sound and the cursor turning blue
to indicate that it worked.

Play as Vermillion and Sho
Do the code above first, then repeat the process with this
combination for Player 2: SK, R2, L1, WK, L2, R1. You should
hear a sound, with the cursor turning from blue to red this
time.

The new fighters will appear in the Random box on the Character
Select screen. Hold Taunt to slow down the character cycling.

For an easier way to enable all fighters, edit this registry
entry:

[HKEY_LOCAL_MACHINESOFTWAREGameBankToshinden2]