Can i use 3.5 mm stereo splitter for this? Help!?

I really wouldn't do it Bob.Your output really isn't designed for dedicated frequency splitting.By all means,have a go at it but please be mindful of possible damage to your Mac.You're better off getting a small cheap 2.1 frequency splitter.I use an 8ch passive mixer which I send audio to active speakers. I'm fortunate enough to have the right crossovers for the hi,mid,low,& subs.Try Radioshack for a small cheap 2.1 passive in/out box.