How can I combine these two amps?

By 'combine' I mean plug them up and together to where they both play at the same time.
I have a Marshall MG Series 30DFX amp with a footswitch input, a guitar input, a line-out input and a microphone input.
I also have a Peavey 5150 half stack
...unfortunately it only has a high gain guitar input, low gain guitar input and an FX Loop input.

Basically,
I want to take a few guitar chords, plug them into each amp to where when I turn both amps on, I hear both amps playing.

Anyone have some ideas as to how I can do this?
ANY IDEAS would be helpful.

Thanks,

Marshall