How

You cannot play as both Mario & Luigi at the same time, but you can play as Luigi if you get 120 stars playing as Mario. If you get 120 stars by playing as Luigi, you will get to play in a super-secret galaxy that has another star for you, making the total Star count 241. This information has been posted by other people on this site for previous questions, so I cannot take any credit for this answer (except for being nerdy enough to actually tell people the total star count). You can, however, play a 2-player co-op mode, although Player 1 will be in control of whatever character you are currently using. Player 2 has his/her own yellow, star-shaped cursor that can help player one catch star bits, shoot star bits, hold some enemies in place (also known as stunning enimies), and can sometimes help Player 1 jump the character (if Player 1 jumps at the same time as Player 2 presses A while pointing at the character, you will perform a super high jump).

There are 242 stars, the 2 player mode is called co-star mode and the last galaxy's real name is "grand finale galaxy"

You cannot play as mario and luigi but you can play them seperatly.
Mario you play with until you get all the stars 120 I think then defeat bowser and watch the new ending and you can play as luigi.
The two player is where you have two remote and player too ansd stun baddies and make mario jump and spin in the jump

It's called Co-op mode and for 2nd player, it's just a point in which you can collect star bits and destroy enemies by shooting star bits at them.

Just a point in which you can collect star bits and destroy enemies by shooting star bits at them in 2 player mode

You can unlock luigi, but you can't play as both of them at the same time. The second player can only collect star bits and make mario 0r luigi jump higher.

After unlocking 120 stars then you can play as luigi but not both