Hm... You´re too late. My game is already gonna change the world of RMXP. :=
j/k
You can use a common event that checks if you press two buttons at the same time and then triggers a switch and a changing of the charcter sprite of the first character. Else it turns the switch off and reverts the graphic. Use the switch also as a precondition to change the graphic, so it changes it ONLY ONCE and not all the time otherwise it could cause lag.