Using L/Mono and R outputs on korg to record in Garageband?

[dano2007]

I have a Korg Triton Studio, and I'm trying to record directly into GarageBand on my iMac, using the Line-In port on the back of the iMac.
The problem I'm having is getting it to record in stereo.

If I run a cable from the L/Mono output from the Korg to the Line-In on the mac, it sounds great, plays in both speakers, but just in mono.

So, then I ran two cables from the Korg, one coming from the L/Mono output and the other coming from the R output, plugged them both into a mini stereo adaptor, and plugged that into the Line-In on the Mac, and changed the GarageBand input settings to Channel 1&2 (stereo)...

When I tried that, the recording just plays on the left speaker, and just in mono. In the timeline, the recording shows a left and right track (one above the other) as if recorded stereo, but it just recorded mono.

Any suggestions?