Thanks for the input Kal. This whole concept seems to be a bit of a stumper. I haven't been able to find anything on the web regarding this in any electrical/diy forums.
The reasoning behind my whole idea to use two power sources is to be able to brew back-to-back batches like your 50a setup, though I'm limited by my 30a dryer plug. By essentially running two systems within one panel, I hope to use the 120v 15a source to heat my HLT and the 240v 30a source to run the rest of the control panel.
I notice with your theatre amplifier example that both plugs look as if they're 15a service. Therefore, any fault would not exceed 15a and would run out either 14AWG ground wire.
The dilemma that we're wrestling with for a two power source panel is a short on the 30a side of the panel travelling down the 15a (14AWG) ground.