If you don't run anything stereo (like ping pong delays or stereo enhancer), then both outputs are the same.

In the I/O menu, you can select wether to copy L channel to R, or Sum L+R.

Also, the manual is available online. There is an entire wiki devoted to both units.
Peavey 5150
Mesa Mark IV
Mesa Single Rectifier (Series 1)
Fender Custom Shop Tonemaster
Roland Microcube

-Whitebox OS 1x12
-Port City OS 1x12

Digidesign Eleven RackAxe Fx Ultra