Using 2 condenser mics in garageband with only 2 inputs

I have started a podcast and have been struggling to get the proper set up for recording. I am using 2 condenser mics and a 4 channel mixer that is providing phantom power. I have finally found a way to get my iMac to recognize the mics with the help of some USB audio interface sticks. However, when creating an aggregate device and selecting the USB interfaces on the list, it only allows for 1 input per USB interface. All of the videos I've watched have shown when recording multiple mics on separate tracks, they have up to 4, sometimes 5 inputs. I've created the aggregate device and followed all of the steps, but I can't get garageband to record each track separately even though the mics are on 2 separate inputs. Any ideas?