Canon EOS REBEL T6i Instruction Manual page 186

Custom Wireless Flash ShootingK
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
186
Basic settings:
Flash mode
E-TTL II meter. : Evaluative
Wireless func. :0
Channel
Convenient when you need a large flash
output. All the slave units will fire at the
same output and be controlled to 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], then
take the picture.
: E-TTL II
: (Same as slave units)