I know that this is an old post but just in case someone else runs across it (like I just did) I wanted to clarify.
BowerR64 is correct. If you run both cables from the Marshall DSL100 or the DSL20 (like I have) into the Line 6 V412 cab, the total load will be 4 ohms and thus it will trash your amp. Also, never run the 16ohm single out in combination with either one or both of the other two outputs.
Since the line 6 V412 cab is stereo, you can use a single Input as an 8ohm from a single 8ohm output of your Marshall if you plug an empty 1/4" adapter into the other jack. I just tested the two jacks on the cabinet with my meter and as long as the 2nd jack has an adapter or a cable plugged into it but not plugged into anything at the other end it will work as an 8ohm 2x12 cabinet.
When both input jacks are used on the cabinet, it severs the connection between the two pairs of speakers, effectively making two 8ohm 2x12 cabs. I actually use two separate heads and run stereo with this cabinet. Some may say that is stupid but given the right heads and setup, I don't need to mess with 2 separate cabinets and the sonic separation when using fx like delay is much more pleasant to my ears even coming from a single cabinet. The mixture of both heads (One Marshall and the other PRS or Mesa) coming from a single cabinet at any distance is a sound I played with and adjusted for a long time to get what I wanted and I can't achieve any other way. I should also note that I am using a Helix with the 7 cable method in the loops of the tube heads and the sounds I can achieve are superb, Also, this is my studio setup, I'm getting too old to be lugging all that around for a live show.
