If you're on a Mac you can go into Audio MIDI Setup and create a new Aggregate Audio device and gang your audio inputs together. Apple will sync the devices through the USB Audio driver for you. In your DAW you'd select your Aggregate Audio device and then you can use it as a single device although it's two devices physically.
If you're on a PC, you have to upgrade to another interface with more inputs. ASIO can only see one device at a time.