Hmm... Maybe you are doing it the hard way.
My plan (if it works) is to import my GPS data and create an overlay of the gauges for the entire flight. Create a video file of that. Then bring that into Final Cut Pro X, and use it as the timebase for my video. I can then drop whatever videos from whatever camera feed I like into the mix, as long as the time lines up, it will match.
For example... If I performed say a simulated engine failure 30 minutes into my flight, the gauges will show that in the way of slowing down, and whatever I happened to do next etc etc. I can grab any video feed I like from my 3 GoPros and drop them straight in. As long as the times line up (and that's actually an easy bit) then it will work perfectly.
One trick is to get a moment in time to snap sync all of your videos. In other words... get all of your cameras running and then make a loud sharp sound that all of them can record. It can be as easy as clapping once really loud. (take a time check too for GPS data) That shows up as a spike in the audio stream when you drop it into the editor. Align all of your camera feeds up at that point, and for the rest of the editing experience, everything is on time.
Hollywood has been doing that method for decades. ;)