How record line-level stereo input on GarageBand iOS?

I'd like to record line-level stereo input sent from my Mackie mixer to GarageBand on my iPhone 4S. I tried plugging a standard 1/8" stereo cable into the headphone jack, but no luck. The GarageBand manual suggests that this is possible, and there is one (pricey) cable I've found that seems to handle this, but it claims that the audio input is mono only, suggesting that I may need a cable connection to the dock (not the headphone jack) to handle stereo. I can't find a cable to do this, however...ideas?

Many thanks,

Jim