When you record by connecting two devices together whether in the same room or remotely, the videos are first recorded locally on device and then, post recording, synced to both iOS devices.

This means that during recording you'll always see a preview of the other device's camera, which will look low-res.

When you have finished recording, you need to sync your videos in order to get the high-quality version. This syncing process happens automatically when you stop recording, but if you want to check the progress, Multiplayer icon again.