just a side note, I don't think you need to do what you're doing to get stereo Phatty sound in Ableton Live.
Just connect a single 1/4" audio cable from the line out on the back of the SP, to one of the two 1/4" line inputs on the Fast Track.
On your computer's audio settings, MUTE the inputs of the fast track pro, so you don't hear the direct sound of the Phatty.
In Ableton Live, create an Audio track and then in the I/O section (audio from, audio to) look at Audio From. You want the top box to say Ext Audio or Fast Track Pro or whatever it says that means the Fast Track Pro...
below this is a second box where you select which channel(s) of your interface to get sound from.
By default this is usually set to stereo 1/2... you don't want this.
Click here and select only channel 1 or only channel 2 (whichever of the two the Phatty is plugged into).
If you don't see individual options here for just channel 1 and just channel 2, go into Live's Preferences, on the Audio tab, and click the Input Config button. Make sure "1(mono) & 2(mono)" is selected. That will allow you to set up your Audio track as a mono track with input from only one mono channel.
Live will automagically send this track to the main mix in stereo! You can adjust the panning however you like, put stereo effects on it, etc. All set.
Hope this helps,
Amos