Old thread I know but did you ever get anywhere?
I have recently set up some HD500/dt25 patches that do (I think) what you are looking for. To check, are you looking to get a preset where one of the 5-8 footswitches will toggle between different two amp blocks, basically emulating a two channel amp? Does that even make any sense??!!
The method I use ends up taking a fair bit of processing and effects blocks, so you can't get carried away. I also can't remember the full settings but will look if this is what you are looking for.
Basically, I put Amp Model A in the left signal path and Amp Model B in the right but output both to the same DT amp. I use a volume pedal block in path A and another in path B, before each amp, each set to zero, one turned on and one turned off, each set to be switched on and off by pedal 6 with no expression pedal control. When you press pedal 6, it will switch one volume pedal on (muting the signal as it's set to zero) and the other off (allow the signal through).
Something that looks a little like this -
Volume - Amp A
Guitar - Wah < > Chorus - Digital Delay - Plate Reverb - Output
Voume - Amp B
I found that I can have wah, simple mod pedal, digital delay and plate reverb with the two amp blocks fine but there's too little processor power for a fancy spring reverb or tape delay. The actual volume pedals effects use practically no processor power anyway (I tested this out), it's the two amps that make the difference. Obviously, using the dt25 I am able to use Amp Pre Models to further reduce CPU usage.