Of the GoPro remotes, only the Smart Remote and the Wifi Remote are capable of controlling multiple GoPros at once. And of those, the Smart Remote is by far the best choice--it's newer, more reliable, and has better battery life. And the Wifi Remote is discontinued. So I'm focusing here on the Smart Remote, but the process of connecting multiple cameras is very similar for both models.
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You can get the HERO7 Black directly from GoPro.com at $50 off, which brings it down to $349.99.
But you'll have to be quick--it ends March 23. They're also throwing in an SD card and free 2-day shipping and returns.
You can find the deal here.
The other methods of remotely controlling GoPros can only connect to a single camera at once. That includes the GoPro mobile app and the Remo voice-command remote.
You can connect up to 50 GoPros to a single Smart Remote. While most of the recent generations of GoPro have built-in wifi connectivity, not all of the cameras are compatible with the Smart Remote. Some GoPros can connect wirelessly to the GoPro mobile app but don't let you pair a dedicated remote with the camera. Some recent examples with that limitation are the HERO7 Silver and White and the HERO (2018). But so long as the camera supports connecting a dedicated remote control, you can mix and match different models in connections to the same remote. So you can, for instance, connect a GoPro HERO7 Black, a HERO5 Session, and a HERO4 Silver to the same Smart Remote at the same time. Or any other combination of compatible cameras, for that matter. You'll get a status line at the top of the remote's screen that tells you how many cameras are currently connected.
Pairing the Cameras
The process for pairing the camera to the remote varies between models. Sometimes the process is very similar, such as between the HERO7 Black and the HERO6 Black. But others, like the HERO4 Black and the Session models, use a different process. Rather than lay out each individually here, I'd recommend checking out the how-to guides I've put together for specific models:
There's no bulk way to pair the cameras to the remote--you'll have to do each individually. So it can be a bit tedious. But as you finish pairing one camera, the remote will give you the option to connect another.
But the good news is that once they're paired the first time, it's quicker to connect them after that.
Selecting Shooting Modes
In general, when you change the shooting mode on the remote, it changes all of the cameras to the same shooting mode. So if you change it to video mode on the remote, for instance, it will change all of the cameras to the video mode. There's no way to drill down into the settings further--it will use the options that you have set on the camera for things like resolution, framerate, stabilization, or Protune, for instance. So you'll need to make sure they're set up correctly beforehand.
There's no way to assign a shooting mode from the remote on a camera by camera basis, but there is a way you can have the cameras using different shooting modes. To have one camera shooting video, another time-lapse, and another burst mode, for instance, all triggered at the same time from the same remote. And that is by using the Default Mode feature. This is something you set on the camera in the settings. It assigns your choice of shooting mode that fires up automatically when the camera powers up. On the Smart Remote, there's an option to use Default Mode, which will then switch the camera to whatever default mode has been set for that camera. Again, this is something you'll want to set up in advance.
Things Worth Knowing
You can change the shooting mode via the remote, but the same changes apply to all the connected cameras. What I mean is that if you set it to video mode, all of the cameras will switch to video mode--you can't assign one to video mode and another to time-lapse, for instance.
In general, the responsiveness of the models to remote commands is similar, but it's not perfectly in sync. Having used remote controls with many GoPro models, I've found that there's a bit of a lag between sending the command the camera responding, and different cameras react more quickly than others. So it's not a reliable way of getting frame-level synchronization.
I'm not aware of a off-the-shelf dedicated synchronization box such as the one Sony has built for the RX0, but a good option that is more convenient and more precise than the old-fashioned slate method is the SyncBac Pro from Timecode Systems, which synchronizes the video timecodes precisely. I have seen some custom setups being used, but they're not off-the-shelf solutions and not something you can just go and buy.