Canon EOS 7D Mark II Instruction Manual page 278

Using Wireless FlashN
Fully Automatic Shooting with Multiple External Speedlites
Multiple Speedlite slave units can be treated as one flash unit or
separated into slave groups whose flash ratio can be set.
The basic settings are shown below. By changing the [Firing group]
setting, you can shoot with various wireless flash setups with multiple
Speedlites.
[1All] Using multiple slave Speedlites as one flash unit
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Basic settings:
Flash mode : E-TTL II
Wireless func. :0
Channel : (Same as slave units)
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].
Take the picture.