No difference in the output jacks. It's just so that you can drive two cabinets. Set the amp impedance to match that of your cabinet.
For running with two cabinets:
Both cabinets should have the same impedance (both 8 ohms, not one 8 ohms and one 16 ohms)
Set the amp impedance to 1/2 of the cabinet values (both 8 ohm cabinets - amp impedance should be set to 4 ohms)
If the cabinets have switchable impedance, set each to the highest value - generally 16 ohms. Then, set the amp to 8 ohms
Install VERY LARGE earplugs, dime the amp and hit an A chord - you are now Angus Young, and the police will show up soon. 
Guitars: '79 LP Custom, '01 Dickey Betts Goldie - 80 of 114, '00 Chandler Lectraslide
Amps: '00 Marshall 1987x, '70 Marshall 1959 SL, 4x12 JBL D120s
'64 Vibroverb - JBL D130, '66 Super Reverb - CTS Alnicos
'77 Peavey Deuce (great for melting stubborn ear wax)