Cutting out all of the detail - A quick overview (that is slightly wrong)
When moving, the front brake will cause the all 3 calipers to apply braking force - using only 2 pistons in each caliper.
The rear pedal will cause braking force to all 3 pistons in the rear caliper, and some braking force to just the centre pistons in both of the two front calipers.
Now for the detailed explanation - (come on, you didn't really think I was going to stop at a short answer, did you ?)
If the bike is stationary, no. Squeezing the front brake lever applies the front brakes only - by pushing out the two outer pistons of the front calipers - both equally and at the same time.
If the bike is moving, yes. The result of the left hand caliper grabbing hold of the spinning rotor is that the left hand caliper bracket rocks forward. This activates the Secondary Master Cylinder which applies pressure to the two outer pistons of the rear caliper.
Because this may otherwise create too much braking force on the rear wheel, and induce a skid (especially since the braking force at the front will make the rear end lighter), the brake lines to the rear outer piston pass through a proportional control valve (located in front of the right hand side ignition coil). When the pressure increases, it overcomes the strength of a spring in the PCV, which moves a spool or piston inside, which acts as a valve to reduce the pressure to the rear brake caliper outer pistons.
When moving, applying the front brake levers results in just the 2 outer pistons of all 3 calipers being applied. The centre pistons do not move. So using just the front lever will apply 6 out of 9 pistons - 4 directly, 2 indirectly.
If the bike is NOT moving. If you use the rear brake pedal, then the centre piston is activated on the rear caliper - unrestricted.
The rear brake master cylinder also activates the two centre pistons in both front calipers. So applying the rear brake pedal activates all 3 centre pistons....
BUT - there is no anti-dive feature on the front forks. Instead, the braking force to the front calipers is limited INITIALLY to just the centre piston in the front left caliper
UNTIL - the pressure from the brake pedal increases to a point where it overcomes the pressure of a spring in the delay valve (at the rear of the front right fork). When it does, the pressure from the rear brake pedal operates the centre piston in the front right caliper as well. This helps to prevent the front forks from diving when controlling the bike with the brake pedal during slow manoeuvres.
Also - when stationary, the SMC on the front left caliper will not operate. Nevertheless, the fluid is able to move past the rear of the primary seal inside the SMC and apply the rear outer pistons (subject to the limits allowed by the Proportional Control Valve).
If the bike is moving. Then the same thing happens - more or less. But now, because the front left centre piston is grabbing the disc/rotor, the SMC moves and it applies the rear caliper outer pistons (again subject to the proportional control valve). So when moving, pressing the rear pedal hard will apply 5 out of 9 pistons - 3 directly (1 delayed), 2 indirectly.
Whether or not the bike has ABS makes no difference to these scenarios. They both have the identical set up. It is just that if the bike has ABS, it is there to repeatedly remove and reapply pressure in a rapid cycle, should the front or rear brakes lock up. But the balance is so good, I find that very rarely happens.
I hardly ever use the rear brake either, except to give it a bit of exercise every now and then. It certainly improves the retardation when I use both together.