Thanks to phil_m for the quick response but it didn't completely apply to me...some have commented that you can set input 2 to Variax, which seems like it works, but the kidney bean HD 500 doesn't have this option.

I just spent some more time with it. Maybe this was obvious to others, but I just spent an hour A/B-ing some test ideas and I figured this out.

As we all know, inputs 1 and 2 are summed to mono if you put any mono FX or a single amp block in pre-mixer block position...this is straight from the L6 manual, and my tests confirm it. This creates all kinds of problems/questions about just exactly how you can be sure you're getting unity gain into a mono path amp model's input. When you fire up the HD for the first time and play around with some dry amp sounds to test the realism, how do you know if you're hitting the pre/drive section of the amp with a "clean" guitar (unity gain) signal? Otherwise you can never be sure where any unwanted compression is coming from in the tone.

What they don't tell you in so many words is, you can create a parallel path DIRECTLY from the inputs simply by moving the amp and pre-amp effects into a SINGLE parallel path chain. No special voodoo here, just grab them as "move" blocks and fiddle with the direction buttons until you get both your amp and pre-amp FX blocks as follows: 1. pre-mixer, and 2. on the same parallel path (top path, for example). At this point, the hypothetical parallel amp #2 will be shown on the edit screen blacked out as "disabled".

So, what you have now is the following:

1. Input 1 (guitar) feeding whatever effects and amp block you have on that path (top path) and finally into the mixer...which is effectively exactly a complete mono path with a unity gain input from guitar all the way to the amp's input, with the only gain staging being what you want it to be (e.g. an overdrive pedal in between guitar and amp)

2. Input 2 (guitar or mic, doesn't really matter for this example) feeding the bottom parallel path through a disabled amp and into the mixer.

Then you just reduce gain on mixer path B all the way down to "mute", and set gain on path A to 0 dB and pan path A to center. Now you can be sure you have unity gain all the way through your chain (except your amp settings, of course) - and more importantly, you can be sure that you are effectively re-creating the same signal path as if you plugged your guitar straight into stomp boxes and then into the amp input, as with a normal guitar rig.

Also, whatever stereo effects you put towards the end of the chain AFTER the mixer block (and mono effects that may happen to be immediately after the mixer, like, say, LA-2A) will still be stereo output (albeit fed by a mono source - mixer path A panned center). This is effectively the Bradshaw/Lukather/Van Halen setup from the 80's...you can have ping-pong delays, or modulations, or whatever stereo effects you want leaping back and forth between monitors/headphones L and R.

On the other hand, be aware that if you put a stereo output effect POST amp but PRE mixer, then you will be feeding that effect's stereo input B from your bottom parallel path (w/ disabled amp), so if you have path B's gain set to mute, you won't be able to hear the left output of the stereo effect (you'll just be hearing the right side panned mono, but panning is a separate issue in this scenario). If you raise the mixer's path B volume in this scenario so you can hear the stereo effect, you'll be hearing the sound of the DRY guitar's other channel feeding the effect's processing path. So in one side you'll hear the sound of the amp feeding one channel of the stereo effect, and in the other side you'll hear dry guitar feeding same effect (could be cool, but probably not!) - unless you enable amp B, which is fine, but that may or may not be what you wanted, plus having both amps enabled is a real DSP hog).

But back to the main solution...as long as Amp B is left disabled, you're not eating any extra DSP by doing it this way, as far as I can tell. Remember if you enable Amp B to test some dual amp sounds, you won't hear the 2nd amp unless you unmute it in the mixer block.

This is REALLY un-intuitive. Guitar players aren't used to thinking like this. I didn't figure it out until it hit me that the dual input paths are making their way through the signal chain whether I like it or not and whether my preferred way of setting up a chain can handle it or not. When you look at the edit screen, if you want a standard, conventional mono path, it just seems logical to arrange the blocks as a straight mono path and let the mixer be wherever seems most convenient. But the way the inputs are set up won't allow this to work the way you think it does.

Geez, why don't they just put a "Guitar-MONO" option in the input selection indicator which automatically disables the 2nd path???? Doesn't that seem so much more logical? Kudos to Line 6 for making a great product, but people's ability to use simple logic and effective communication is generally getting worse by the year. Thank your local Teacher's Union for making sure that at least 3 generations of Americans have now managed to make it through our public school systems without ever learning how to think critically or communicate ideas effectively.