Sync video recording from two iphone 6 or 6+
It can be done, yes. However, it's somewhat involved.

One way would be start filming on both iPhones and then play a high-pitched sound of some kind. Then, later, you can use video editing software like Adobe Premiere, or Final Cut Pro (or even iMove, I suppose) to visually sync up the videos by looking at the audio tracks of both videos and aligning them up visually by looking at the sound spike created by the high-pitched sound played during recording.

This is kind of like what video professionals do when working with multi-camera setups. Not exactly easy. But technically possible, if you know how to do video editing.