Canon EOS REBEL T7i Instruction Manual page 224

Custom Wireless Flash PhotographyN
Fully Automatic Shooting with Multiple External Speedlites
Multiple Speedlite slave units can be fired as one flash unit or separated
into slave groups for shooting with flash ratio control.
The basic settings are shown below. By changing the [Firing group]
setting, you can shoot with various wireless flash setups with multiple
Speedlites.
[1All] Firing multiple slave Speedlites as one flash unit
224
Basic settings:
Flash mode
E-TTL II meter. : Evaluative
Wireless func. :0
Channel
Effective when you need a large flash
output. All the slave units will be
controlled to fire at the same output and
obtain a standard exposure.
No matter which firing group (A, B, or C)
the slave units belong to, they will all fire
as one group.
Set [Firing group] to [1All].
1
Take the picture.
2
: E-TTL II
: (Same as slave units)