Hi

On their own, currently, the only way to do something like this is to set up a dual chain with two independent mono configurations and two amp models which does impact on the available DSP resources.  It's do-able and it works, but it's not my preferred solution.

I found that as long as I was satisfied with relatively simple speaker cab simulation, I can use an inexpensive active DI box with switchable speaker cab emulation to feed the PA in the form of the Behringer Ultra GI GI100 DI box (or pair).  These retail at about £25 GBP and are a cheaper alternative to something like the H&K Red Box.

The result when placing the DI box between the HD500 and the amp input is in my opinion a more than satisfactory and usable sound through the PA whilst feeding an on-stage guitar amp.

In practice having access to both a DT-50 212 and a DT-25 head + cab, I would normally just take a PA feed from the amp's speaker transformer derived DI output or if using a third party amp, I'd place the Behringer box between the amp and its speaker to achieve the same thing. 

If however, you want to use a full amp model for the PA and PRE for your on-stage sound, you are really back to using a dual signal chain as I outlined in the first paragraph.

Or yet again, as a quite expensive solution, given that the HD500 is a MIDI controller and has digital output, you could buy both an HD500 to act as MIDI master and controller with full amp models and cab emulation to feed a Studio/Direct signal to a standard third party PA desk and MIDI control/digital audio; and a POD HD Pro to allow the POD HD Pro to be set up in parallel with the POD HD500 and essentially with the same patches and set lists as the HD500 but using PRE amp models and with the output mode configured to Stack Pwr Amp or whatever mode suits your on-stage guitar amp.  This is probably not a solution most people would entertain, but with the recent addition of MIDI control that can be used with incoming MIDI signals in the POD HD500 and POD HD Pro units, it does make the POD HD range scaleable and therefore possible to run several POD HD Pro units in parallel.

Nick