From 737 QRH:
"..............
Engine and APU fire switches(All).......override and pull
If an engine fire light is illuminated:
Related fire switch...........Rotate and hold.
Rotate to the stop and hold for 1 sec."
This is the new QRH ,march28/2005.
It should be the same on the 757 ,i guess.Previously ,you were supposed to fire bottles in both engines,now only if you have fire.
Brgds Alex