You need ActionReplay Version 3.2 or higher to do this cheat:
Use these codes to get infinite BP(BrotherPower)
&HP: MarioInf.HP MarioInf.BP
995fa0d9 655a6f06
0c6720d2 5ac60038

LuigiInf.HP LuigiInf.BP
4772acf2 a2db8132
47b4675b 4069bc9e

NOTE:The 0's are zeroes,&the letters are capitalized when you put the code in.And one more thing:when you use these codes on your action replay,(including m)don't turn all 4 of them on.If you do your game will freeze up!