The Webcam Tool available here supports multiple GoPro USB webcams. This means it is possible to use multiple GoPro cameras as webcams on a single computer, for instance in OBS Studio. The tool is available for macOS and Windows.
The official GoPro article on the GoPro website says that it is not possible to control multiple GoPro cameras with the Quik app at the same time. But there is a solution for this problem.
Since version 1.3.2 the mobile (Android, iOS) version of the Camera Tools app supports the following keyboard shortcuts which can be used with a Bluetooth keyboard:
Fortunately great alternative apps exist which allow you to use your GoPro from a Mac, desktop PC, and Raspberry Pi. Most tools are also available for Android and iOS.