it's unfortunate you are trying make a sunn sound like a marshall. Putting this aside, what you need to run both is a sophisticated splitter box, AB boxes work but you could suffer from ground hum/earth loop interference. What you really need is a splitter box that has a phase shift as well as a ground lift for each channel, then you will be sorted. I have a one in six out custom made splitter box with ground lift and phase shift and volume control for each channel and it works a treat when running six amps at once.