You could use two iMics to get two mics into SignalScope. However, you would have to make the two iMic input devices into an aggregate device in Audio MIDI Setup.
Depending on what kind of mics you use, and what their power requirements are, if any, it may be easier just to route one to the left input and one to the right input of your Mac's built-in audio input jack. If, for some reason, you can't use your built-in input, you could use a single iMic in the same way. Two inexpensive Radio Shack electret mics (typically battery powered) and an appropriate adapter could do the trick nicely.