I wrote the same thing in your other identical post in case you are only coming back to only one of them.

I suggest to simply do the following...which I have written in my Start-Up guide for the POD HD500 posted on the main POD HD group page if you wish to read it...

• Try plugging in a pair of headphones and see if you hear the same result when shutting off Amp A or Amp B.  If you hear both amps in the headphones but not through your amp - then either try what has been already suggested to sum/mono your dual amp signal, or perhaps - with headphones on, try going point by point with all FX off (preferably take all FX out and leave only the two amps just for this testing process.  No need to save until you find where the routing problem lies and you correct it.  Then put back the FX - check your patch again to see it is still working as you expect, then save that patch and go onto any other patches from there.  
• With just the two amps in your signal chain try doing one on, one off and hopefully you'll be able to rack down where the signal path is getting sent and why.  Off = A  On  = B  - & vice-versa and see how it sounds in the headphones.  
• If you are still hearing only Amp A only and not amp B when Amp B is alone...then it's probably a setup  issue most likely found within the 10 main pages found when holding down the View button on the unit. 
• This is my first guess to the cause of your problem...Do you have your amps not only on the mixer, but in the AUDIO OUT control section set to Left/Right, or to something else like Amp 1 = Amp A, just Left, etc.?

I hope this helps and you get your signals running the way you want them.

Take care,

Neal