I don't remember if it was a specific button (-, +, A, B, etc.) but after the first person
starts playing, the other can join in at any time.

btw, can you tell me how to save the game in the midst of playing so that i need not
restart the game from the beginning evertime I stop half way

You cannot save in the middle of a level, but it auto saves when you beat a level. It also
has built-in check points so if you die, you only go back to a certain point.

If you play 2 player, it is a lot easier because you are only considered to have 'died'
when BOTH players die. If one makes a mistake or is overwhelmed by monsters, they will
come back in a few seconds (as long as the other can stay safe). There are a few jumping
puzzles where 2 player doesn't help because neither player would survive if they waited
around (i.e, falling platforms, timed obstacles).

How do you fight the energy core at the plumber base? I can't get pass this level. Any
suggestion?

Even if you are the same person, I wish you would ask the questions in a separate thread
(of course then I probably wouldn't be notified that you asked a question... but that
would be CheatsGuru's fault).

If the energy core is the part I'm thinking of, then you just roll up the ramps as
Cannonbolt destroying all the little things on the wall. After you destroy the accessible
ones, a different ramp will raise. At least one of the ramps gives you access to multiple
rows of the tings on the wall.

I sat there myself wailing away at the core, but nothing happens. Just go near the door
and I think it opens.