I just used 3 cheap HD Cams left/right/center (you can set the left/right to close up) then mix the recorded audio from the mixing deck with the video after, just turn the volume down on the video tracks and leave the recorded mixing deck audio track up. Might be easy to record the mixer audio directly into Reaper then add the video tracks after.
Here's Reaper-Multicam-Mix.mp4

Download Multicam Videos here to practice (Download button under videos)
https://vimeo.com/110045836

MULTICAM RAW FOOTAGE - Jazz song Cam 01
MULTICAM RAW FOOTAGE - Jazz song Cam 02
MULTICAM RAW FOOTAGE - Jazz song Cam 03
MULTICAM RAW FOOTAGE - Jazz song Cam 04

Guide for Reaper