You have 2 sets of 2 speakers, yes?
You want one set to be the front left and right channels, and you want the second set to be the surroud left and right channels, correct?
You plug one set into the Front Output (green) on the SBLive!, and the 2nd set into the Rear Output (black).
You then proceed to put the two front speakers in front of you and the two rear speakers behind you...and you will get 4 discrete channels.
You'll have some volume balancing to do, but that's not that hard.
This stuff about the splitter will make you have two front Left and two front rights, with no rear (surround) channels.