I have successfully set up two iPhones, a 12 and a 14 that I manually start slow motion recording, one first then the second one.

I cut out a certain part of both (roughly the same content) and airdrop it over to an iPad and then I import it to an analysis app (CoachNow). I was surprised to find that they do not sync in time, one of them seems to have double frame rate. Both phones support HD slow motion recording 240 fps and I make sure to set them to that. Also specs on the cameras on these two iPhone versions seem very similar.

Questions
1: where does the problem arise? At recording time, cutting time, exporting etc? Any ideas?
2: how can I downsample the faster one, if it’s not possible to fix this at recording time?
3: eventually I’d like to produce a video like this, where the two streams would be in sync, what’s the simplest way of achieving this?
https://youtu.be/SwCS90quKHE?si=XASq1Cc54RJdTxK1 (See 2:30 in).

Rather than sitting with iphones/pads I’d like to get it over to desktop, preferaby Ubuntu, but windows would be fine too. What software should I use, cheap is good but I’m willing to pay 50-100 $ if it works well?

Hope anyone would help me in the right direction. Really noob at video editing, but engineer as profession so I fairly easily understand the tech stuff. Thanks!

P.