I use Gnome desktop and have the Unity launcher running as well. I was able to achieve this by installing Gnome and doing this. After I did it, the Gnome panel at the top and the Unity launcher on the left were overlapping in the top left corner. I switched between Gnome, Gnome Classic, and Gnome Classic (no effects) at login screen several times (and installed some other packages including CCSM) and eventually they started playing nice together and the top of the Unity launcher starts under the Gnome panel. After that, I stopped messing with it altogether for fear of them overlapping again. I'm not sure what actually caused them to behave how I want.
Now this hard drive is failing and I've got a replacement being shipped. I want to recreate the setup I have now without all the guessing. Is there a simple set of steps to achieve the desired result (Unity launcher bar at left and Gnome panel bar at top do not overlap in top left corner)?