Area Change in 2 Player Mode:
Both Players must do this at the same time. It is really simple, just go to a flat wall and run against it for a minute or two and it will open up to a new area of that level. Hint, some levels like the bamboo thicket do not work in this situation.

Enable Katze:
To play as the awesome boss with the gun (Katze) beat slash mode on hard without continuuing. He is selectable in 2 player mode only.

The Real Ending:
Do not touch anything on the control pad until you hear the character's weapon click into place. Do not kill him from behind. Do not kill him on the ground. Do not use your subweapon. The Bushido is hard to follow on the gun guy.

The Second Ending:
First, you must have the first ending for the character you are using. You cannot be TOUCHED through this whole process. Okay, run all the way to the hole in the ground. Disable the other guy's legs and jump in. Go through the game as you normally would and you should encounter a second boss after Hanzaki. I think it depends on who your character is