Hey ericksr,
When adding an "On button index pressed" condition, one of the values you can specify is the "Gamepad" number. This number is "0" by default, which essentially means the first player. "1" would be the second player, "2" would be the third player, and "3" is the forth.
So, to create an event that detects only the second player pressing the jump button, you would use the "On button index pressed" condition and enter "1" for the "Gamepad" number.