ok.. so heres the situation..
a squad of guys.. one plasma gun one lascannon lots of bolters and a vet with combi plasma.. they decide to shoot at a tank..
1.) shoot the lascannon see the result, shoot the plasma gun see the result, then shoot the combi plasma... that way you dont waste it if the las or plasma kills the tank.. since you pick the squad shooting, pick the unit.. then roll for the weapons individually..
2.) do you have to declare that the combi plasma is shooting before you roll dice... that way no matter what happens you have either used it or not used it?
it is an interesting question...