Shortcut or badcut?

I don’t think you’ll get air, but hey, so long as both speaker pairs are 8 Ohms, try it. :)

Speaker placement and room acoustics are a better way to go. Experiment for free with some blankets and pillows on the walls and on the floor.

Is there some point to try to add a bit more air to my speakers, using the 'second set of speakers'  switch on my receiver?

The usual meaning of "air" is used as a metaphor for a presentation in which individual instruments and voices are heard so clearly individual and distinctly separate as to have a space or air around them. The sound of each individual musical source reverberates within the acoustic recording space and this too when believably reproduced is referred to as air.

Is that what you're going for?

Buy a pair of JanZen D-130 electrostatic tweeters! These were used in days of yore to improve the treble of Acoustic Research and KLH box speakers. Unfortunately long out of production. I have a restored pair, along with vintage AR's and KLH's!

I assume you have not got separate volume controls for speakers A+B.
If you put the speakers in the back of the room at greatly reduced volume it will make the room sound larger. If you put them up front with the others it will screw up the imaging like a blurred picture. Roberjerman is right. If you run tweeter up above 10 kHz this will give you a sense of air and detail but it is artificial and most of us do not do this. The "air" in the recording is the right air.