Hello,
Hmm, they both should ring if you have an alarm set on both and they are not in silent mode:
See these standard steps for sound issues:
1) Is the side silent mute button on? Does any red show? Slide it back and fourth.
2) How about the volume buttons? Is the volume very low? Move the buttons. Adjust the volume on iPhone
3) Is Do Not Disturb on? Go to Settings > Focus > Do Not Disturb. Is it turned on? Check the times.
4) Go to Settings > Sounds & Haptics. Check the Ringer and Alerts slider. Try changing the sound pattern for the type of alert.
5) Go to Settings > Notifications. Is the app listed?
6) Do other sounds work? What about phone calls? Music?
7) Make sure that the watch is not in silent mode.
Good luck.