Water-cooling stepper motor with aluminum block
Cooling any single face of the motor is fine. The motor case conducts heat very well. Many 3d printers effectively cool the motors merely by having the output drive face of the motor bolted to a metal bracket which is bolted to the aluminum extrusion frame. That's it. That contact alone cools the motor, which is shown by how the motors overheat when people install vibration dampers, which puts rubber between the motor and the extrusion.

The only thing I would worry about is the surface area inside this particular style of block, and ensuring that you use an all-aluminum radiator since the block is aluminum.

There is another type of cheap 40x40 aluminum block with the nipples on the face instead of on the edge. That style has fins and a lot of surface area inside. This style with the nipples on the edge has a big S channel inside and not much surface area. It may still be more than enough. I'm just saying it's a significant difference and it might make a difference. If you find you're not cooling enough, all you may need to do is switch to that other style of block, not try to install 3 blocks per motor or any other exotic nonsense.

But no way do you have to worry about coooling more than one face of the motor, nor does it matter rwhich face it is, especially not with active water cooling like this, as long as the block is actually making good thermal contact with the motor (thermal tape, or thermal epoxy, or thermal pad &amp; zip ties)