After getting interested in podcasting I decided to purchase and use two Blue Snowball Microphones and Garageband 3. The only problem I found is that when I connected both USB mic's at the same time to Garageband I could only use one of the microphones at a time.
So after searching the internet and apple forums I found some work arounds, but finally found a way that works great and is easy to setup using the utilities provided by my Mac.
So here is how I found out how to do it:
Step 1. Plug both of your USB mics into your Mac
Step 2. Open up the "Audio MIDI Setup" utility Applications -> Utilities -> Audio MIDI Setup
Step 3. Using the Audio MIDI Setup program open the "Aggregrate Device Editor." You can access this from the menu under Audio -> Open Aggregate Device Editor
Step 4. Locate your two USB mics from the list under structure and click the "+" button to add them to the top. You can then click on the Aggregate devices and change the name to something you can remember. I changed mine to "Dual Blue Snowball Mics" so that I could easily identify it later on. Make sure you click on the "Use" button on the bottom under structure to ensure those devices will be used.
Step 5. Change your System Settings under your "Audio Devices" to indicate your newly created aggregate device "Dual Blue Snowball Mics." Under the Properties For: make any necessary changes to the aggregate device. You will now notice that one of them is set to use channel 1 & 2 and the other will use channel 3 & 4.
Step 6. Open your Garageband podcast project. You now want to set your Garageband Preferences to take the input from your new aggregate device. Garageband -> Preferences -> Audio/MIDI -> Audio Input and find your device for mine I added Dual Blue Snowball Mics.
Step 7. Now to add the input to the podcast poject. For this I used one male voice track and one female voice track.
First clicking on the male voice I clicked on it to bring up the info in the bottom right of Garageband and made sure it was set to take input from Channel 1 & 2.
Next clicking on the female voice I clicked on the track info and made sure the input was taken from Channel 3 & 4.
After testing this out it worked great. The inputs from one mic went right to a single track and the inputs from the second mic went to the other track.
Good luck and happy podcasting.
The dude abides.