Beachtek has a solution in their DXA-Pocket mixer. In terms of size, it is perfect for a Blackmagic Design Pocket Cinema Camera but it'll work with any camera that has a 3.5mm stereo mic input.
You can connect two microphones with 3.5mm plugs and this will send them to separate channels so that they're recorded cleanly and that makes post-processing a lot easier.
Looks like a simple solution to a problem that we all encounter at one point or another. Through the 10th of November, B&H Photo has the DXA-Pocket for $80 USD