Q Do I need to buy an external audio mixer to merge separate audio inputs on my Mac?
A Provided that your Mac recognises these are separate audio devices, you can create an ‘aggregate’ audio input (or output) device from combined multiple inputs (or outputs), using AudioMIDI Setup (in /Applications/Utilities).
Click the + button in the Audio Devices window to make a new Aggregate Device. Select that, and then check the Use boxes of the listed devices, that you want to include in the aggregate device. If you want one source to act as the master clock, select that input at the left and bring up the contextual menu so that you can select the Use this subdevice as master clock option.
Audio applications such as Logic Pro and MainStage have their own mechanism, detailed here.
Updated from the original, which was first published in MacUser volume 26 issue 8, 2010.