Timecode Systems UltraSync Blue, Firmware v.5.11 and BLE 5.04
A pair of iPhone 6s (iOS 15.8.3) running Mavis Pro Camera version 5.6.2 (1168) (Last version available for the old phone.)
Moondog Labs Anamorphic Lenses. 1080 30fps. (I didn't know I could shoot at 60fps with the Quicksync Blue)
I'm really pleased with the sync as it is frame accurate because of the timecode recorded with both video clips. The choppiness of the motion is due to higher shutter speeds and of course 30fps. (Update, you can shoot 60fps, just select the frame rate from the app itself while timecode continues to be fed to both phones).
Shooting at 1080 means giving up resolution. :(
Notes: A ND filter would help to smooth out the motion. This is the way forward with using two iPhone to record 3D. I'm now window shopping for another pair of iPhones. (iPhone 12 with the straight camera layout should do nicely).