Can you dim a dimmer?

My room has a day night thermostat. I have a industrial dimmer (see photo) plugged in to the thermostat and my blowers plugged into the dimmer. My blowers are overkill at 100%. At 70% they are so quite too.

One blower blows more than the other so I want to add an inline dimmer to one fan. Which means it will be plugged into two dimmers. Is that bad? One dimmer is the one seen in the photo and the other is a 4$ home depot dimmer. I will dim it until the room pressure is the same as outside the room. Right now its at a positive pressure which is not bad because bugs can fly in.
I guess I could plug each fan into its own dimmer but id still like to know if I can use two dimmers on the same fan?

Also do dimmer waste energy?