First take one amp. Use FX Send on the master amp to FX return on the slaved amp. Then you have 2 sounds, the master amp is feeding its sound through the power amp of the slaved amp, which will alter the sound. And of course you need 2 different cabs so that will also alter the sound of the original amp.
The trick is to get 2 sounds that compliment each other. One sound can be a little bit brighter, the other one a bit duller, and maybe one is thicker and the other a bit thinner, you get the idea
the thing is to have the 2 sounds in mind when setting the tone, which will lead to an awesome tone in the end. I usually have one guitar about 5-10db louder in mix. The one that is a bit lower i usually pan 80. So it is Master amp 100 L/R , Slaved amp 80/80.
The tone gets much thicker and wider if you are good at this technique which i learned at Studio Fredman when i was an intern over there. I'll see if i can remember to bring some samples tomorrow from the studio.