This is a limitation of SBLive. It has one stereo Line In to record, and one stereo mic Line(crap). Only one of these two stereo inputs can be used at a time.
To record multiple device to multiple tracks simultaneously, you would need a different soundcard. Using a mixer, you could record two separate MONO tracks with two separate instruments by panning one hard left and one hard right and setting up n-track as mentioned above, to record two mono track intead of one stereo. Ultimately, you will eventually want to get a more versatile soundcard. SBLive works very well(I use one right now) but I am quickly becoming aware of its limitations.