Garage Band was not designed to do what you are trying to accomplish. You need to graduate to better tool for this. Garage Band is designed for simple audio input (your Mic input on your Mac) or a USB Audio microphone for example.
Your StudioLive 2442 comes with a free copy of Studio One Artist which will allow you to record all the inputs at the same time. There you can record all your tracks. Then go back and re-record more while muting others and sending audio to different aux's as needed.
Looking at your account I do not see the StudioLive 2442 registered. Take a moment to register it by logging into your account at http://my.presonus.com
. Upon registering the mixer, Studio One will appear in your account. You can then download it, install it and start using it for what you are trying to accomplish.
You can also use Capture software available in your my.presonus.com account or on our download page here: http://www.presonus.com/support/downloads/Capture
Using Capture, it will record all the inputs (like a big tape machine) to a single folder, there you can find the stems that you can then import into Garage Band.