My son does audio work. It's his paying job. He tells me that to solve this he uses a mixer. But that's not his preferred solution. His preferred solution is to have an audio recorder then mix it all back up in post production.
Hope this helps,
I currently got a HF M50 with 2 mic's connected.
First is a handheld mic that is connected into the "Mic In" on the camera.
Second is the Canon DM100 mounted on the top of the camera.
For the past 2 weeks I have used the "Mic In" only and it grabs audio directly from the mic (good quality) but the audience sometimes responds and talks with the speaker, and they are not heard.
So I added the DM100 on the top, then all of a sudden my Audio Source on the camera's settings changed to grab audio from the Shotgun Mic only and not from the cable inserted in the Mic In.
When I had the handheld mic connected by itself, I could use the Audio Mix feature which can make me adjust how much % of audio it grabs from the handheld mic and camera internal mic. As soon as I added the shotgun mic, that feature got disabled and it grabs from Shotgun Mic only.