Merging Timer Footage with Gameplay Footage (?)
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In the future, why not run the timer on your Mac? LiveSplit may not (or it might, I dont' know) work on Mac, but surely there's a timer program that will. Then you can record it all together. As far as I know, most of us are doing it this way as opposed to merging videos.
As far as merging, again, I would just suggest picking one of your systems (probably the Mac) and replay the video with a new timer running. Then you can record the video & the timer together in one new video.
Merging videos is an overly complicated solution IMO.
LiveSplit isn't running on Mac yet, but it's planned.
Plus, if you want a Mac timer, look there, there is 4 differents timers softwares running on Mac : https://kb.speeddemosarchive.com/Mac_Recording_and_Streaming