^Wow that's quite a setup. BTW, do you use Audyssey with A, B or A+B?
The 4311 indeed has an old-school Front A/B Amp Assign setting that allows switching A/B/A+B. But it utilizes the int amps and the speaker terminals on the 4311. But I think what you're asking for is two sets of F R/L preouts with separate control of the trims for A and B Fronts. But I don't think that can be done in the 4311 and I don't know of a processor that does that.
Here's an idea that might work: Y-split the FR/L preout signal from the 4311 and either
1. put an in-line attenuator of an appropriate value on the ICs going to the ext amp channels that will drive the Klipsch. Or
2. run that Klipsch stereo pair thru a nice little preamp with vol control, then on to the ext amp. The other pair would run straight to the ext amp.
I'm pretty sure you can split the line level preout without a problem. Audyssey will up the ch trim to compensate and there should be no appreciable loss of SQ with good Y cables and good attenuators. You would place them right on the IC from the 4311 to the ext amp. The challenge is they come in certain values and you may not be able to find a value just right for balancing your speakers.
One way to determine the desired value is to do a single mic pos Autosetup calibration with just Front A connected. Note the value of FR/L ch trims in the AVR. Rerun with just Front B connected. Subtract the new FR/L trims and the diff is the value of attenuator you'll need. (Prior to doing this test use Network Save to save your current calibration so you can Network Load reload it afterwards.)
Another probably slightly less accurate but easier way is to turn off Audyssey and run the AVR int test tones, using your SPL meter to compare just Front B connected vs just Front A.
Here are some suitable attenuators:
There is some discussion of these attenuators you'd need on the Audyssey thread and conveniently summarized here in the FAQ:
which is where I found those links.
edit: hadn't seen your edit. Yup, those.