This seems to be coming up a lot lately.  Please read my following reply and ignore the following criticism by the person who doesn't appear to fully understand how the output modes work when cabinet modeling is not in use.

http://line6.com/support/message/342237#342237

Basically you run a dual-amp configuration two independent parallel paths (configure the mixer block to hard-pan each path to its own channel), one dedicated to FOH and the other dedicated to your combo amp.  Run the amp model driving the path eventually feeding your combo with cabinet modeling disabled.  You should probably consider just using a preamp-only model for this path as well unless you prefer the sound of full amp model and have available DSP resources after configuring the other path for FOH.

If you're sending XLR to FOH and 1/4" to your combo amp then you need to insert a dummy plug into the unused 1/4" jack.  If you don't then your combo amp will be sent the mix of both paths. You don't need to worry about the XLR mixing both channels, since it doesn't behave the same way as the 1/4" outputs.  Of course you can only run the one channel that you tailored for FOH.

Now this is assuming the combo amp you'll be using has an effects return (power amp in) so you don't have to use its preamp.  If you have to run into the front of the amp then you'll need to use the cleanest channel and set the tone controls in order to get the best sound (begin by setting the bass and treble to minimum and then increase the middle to taste).

Your other option is to set up the HD500 to drive your combo amp and then use a separate DI box with cabinet emulation in order to feed FOH.