Shortcut or badcut?

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. 

I would suggest using a pair of Townshend Super Tweeters. If they don't accomplish "air" they will still enhance your current speakers or for that matter most likely any speaker.
I have used my Townshend Super Tweeters with 6 different pairs of speakers with very good results.

@sound22card - If you think about what is at play IF you were to do this...- 

e.g. Each set of speakers has it's own cross over, so unless you implement the same model/year of speaker there will likely be differences in the signal path, which can lead to degraded sound

Different speakers will have different tones, so now the entire tone has changed and maybe not for the better

The two sets of speakers could result in out of phase signals - which will impact sound/image - sort of like a bad echo effect

These are just a few of the issues.

It really depends on how picky your are about your sound.

The "air" I currently enjoy is due to the speaker cables and the speakers I currently have.

The speakers/crossovers are tested and tuned in an independent lab to extreme levels in order to deliver the best quality sound their drivers can accommodate and the cables allow them to function to their highest level.

I would start with cables

What cables do you currently have???  

Regards - Steve