To use 2 audio interfaces with Logic, you will need to setup what is called an "aggregate device".
Here's the steps required to setup a new aggregate device :
1-Go to the "Utilities" folder and open Audio Midi Setup
2-Select the "Audio Devices" tab
3-Go to Audio Menu and choose "Open aggregate Device Editor"
4-Click on the "+"
5-Check the boxes of all the audio devices you want and give a new name to your device by double clicking on "Aggregate Device".
7-Select the "Aggregate Device" as your new "Default Input". Leave the Output to your main audio interface and quit "Audio Midi Setup".
8-Open Logic and go to Preferences / Audio and choose the new "aggregate device".
Tell us if you have any more questions!