I oftenly do some video with EOS M too. I own a cheap shotgun mic and it makes everything easy since sound is recorded into the same video file.
However; for the better sound control, I use external sound recorder to handle of the audio part and sync it with video in some editing software later.
Recently, I just did an Interview Video. I used EOS M as my main video recorder and EOS 7D as the second video. For the audio part, I used a Lavalier mic (shirt clip tiny microphone) with a Korg portable mixer (very old one). After that, I put them into a video editting software and worked from there.
For your information; Zoom has some good portable sound recorders (H1, H2n, H4 and H6) and can handle situations like this very well. I just got myself H6. It works pretty good for me.