Orbot tries to kill Tor processes it doesn't own
Orbot fails to start when Briar is running because Orbot tries and fails to kill Briar's Tor process, leading to the following error in Orbot:
Waiting for control port... Unable to start Tor: java.lang.Exception: Cannot kill: /data/data/org.torproject.android/app_bin/tor Set background service to FOREGROUND TorService is shutting down An error occurred stopping Tor: Cannot kill: /data/data/org.torproject.android/app_bin/tor Something bad happened. Check the log
It looks like TorService#killAllDaemons() is taking its name a bit too literally. If Orbot had been running as root it would have succeeded in killing Briar's Tor process.