garageband

I am starting a podcast and I wish to record it on Garage Band. I have 2 sets of headphones and two USB in Mic's. I have a splitter for the two headphones to line in to the Mac OR I can jack straight into the Mic's. What I'm unable to do when my headphones are plugged in is hear my co-host talking to his mic on my headphones and Vise versa. is there a quick fix? Settings issue I'm unaware of? Any help would be appretiated folks