Canon SPEEDLITE EL-1 Advanced User's Manual page 207

P.Fn-03:
When performing linked shooting (
to the camera. Set it on each flash to be used in linked shooting.
0: OFF (Disabled)
The flash does not fire during linked shooting.
1: ON (Enabled)
The flash fires during linked shooting.
Caution
If you fire multiple Speedlites simultaneously during linked shooting, the
appropriate exposure may not be obtained or uneven exposure may result.
(Flash firing during linked shooting)
), you can set whether or not to fire the flash attached
207