Two Inputs on GarageBand?

How do I have two inputs on Garageband? I'm trying to have one for my guitar amp (connected by USB to USB-C Adapter) and one for my voice which I use the Apple Earpods 3.5 mm jack plug.

I first add my guitar amp (Spark 40) (input) to my output (Apple Earpod wires):

Then I go to add my voice, which I use the mic on the same Apple Earpods as the export:

And then, the amp input can't be found. Only the Apple earpods are the input.

I have not been able to figure this out for years. I've watched every youtube video and read every discussion on Apple. I'm going to go crazy! Please help. And yes I know Configure Track Header -> Record Enable but that still doesn't change the input issue.

I have a 2020 Macbook Air. My amp is plugged into the USB-C port to the left of my laptop and my apple headphone wires are plugged into the 3.5 mm jack to the right.