Sounds like you need some help with your hardware.
This page may help:
Be sure to scroll down to the section where it shows you how to configure your hardware inputs and outputs.
If you don't see your second AudioBox input, it may be because you haven't added it to your Song's configuration.
If you are looking for how best to mike a guitar cabinet, Sweetwater has a nice video:
Adding a second, third or fourth microphone is not much different than when you set up your first.
microphone -> mike cable -> audio interface
Audio Interface channel configured in software settings
New track configured to record from new channel