can both balanced and unbalanced be used a once?

Most balanced outs put out a 3db louder signal and IMO it would be a mismatch.

A lot of preamps with both balanced and unbalanced outputs can drive both at the same time. I doubt that you'd hurt the unit by trying it. That said, the two different amps could have different input loading and sensitivity specs, which could lead to one amp playing louder than the other. That, in turn could skew the frequency balance of the speakers. Still, you could try it and see how it sounds. If it sounds alright to you then you should be good to go. Since these are all Classe pieces, one call to the manufacturer might be prudent...

I have been interested in trying this myself except coming out of an electronic crossover rather than the preamp. My preamp says the balanced outs are 6db louder.

My electronic crossover manufacturer says it can be done, but it also has gain controls for matching.

Mixing two different amps even from the same manufacturer can be tricky business. While it's possible that different out puts could compensate for it, it's much more likely that it's going to further complicate things. Might be worth considering replacing your amps with something more manageble.

I have a Classe 30 (currently sitting in the box, because of upgrading), and I used the balanced and unbalanced out at the same time for several years -- balanced to drive the main power amp, and unbalanced to drive a powered subwoofer. I experienced no problems whatsoever, though eventually the upgrade bug bit. It is true that you will likely have an issue with balance between the amps, though, if neither has level controls. Another option, which I also used with the Classe 30, is to split the unbalanced output using y-connectors (I used some nice heavy goldplated ones sold for car audio); then both amps are getting the same signal. I'm sure the same thing would work with the balanced outs if you can find or make the appropriate y-adapters. Anyway, I agree that you won't hurt any of the gear by trying it. Good luck!

I use balanced and unbalanced outputs to power one system in my office and the other in my living room. As many above have stated you will encounter problems using them in the same room. It might not be worth it.

One thing not to do: try and get fancy and use balanced and unbalanced inputs on the same amp at the same time, using the switch to go back-and-forth from one source to another.

Many amps double-up circuits and simply moving the switch back and forth will not isolate the systems.

Thanks for all the comments guys. It gives me some clarity on the various issues involved and possible options. From the sound of it no harm would be done by experimenting with using both sets of preamp outs simultaneously. Whether it will be beneficial, we shall see.

I emailed the manufacturer on this topic after hearing from a few of you as well and will let you know what they have to say.

You will find Classe' customer service to be very responsive.

To confirm most of the previous comments: The guy at Classe responds saying there is a 6 db output level difference between the balanced and RCA connections and for that reason using both simultaneously is not advisable.
He too suggested the use of a y connector to either one or the other set of preamp outs.