I’ve yet to really get stuck into it, but the cameras have a linux subsystem on them that expose a web server and ways to prod the camera into doing stuff.
http://nutseynuts.blogspot.com/2015/06/xiaomi-yi-action-cam-custom-scripts.html has a bunch of scripts to give examples.
By default, the camera hosts a wifi access point, expecting your phone to connect to it. There’s scripts available (like here: https://dashcamtalk.com/forum/threads/camera-wifi-in-client-mode-working-ash-script-inside.12239/ )that make the camera connect to a wifi access point. From this point, it may be possible to simultaneously(or near enough) send commands to multiple cameras on the same network.
Thus far, I’ve only used the following script in a file called autoexec.ash, that consistently yields a photo every 2 seconds, despite the 1 second sleep:
t app key shutter
As I typically walk whilst capturing, this frequency is sufficient, although I may experiment and see if it can reliable manage 1fps to match my phone.