How can I stop my paired Apple Watch from alarming?

i have upgraded my original iwatch but still want to have my original one charged to wear when my new one is running low on battery. The watch app automatically switches between the two which is fine for day to day use but the alarm keeps going off on the spare one waking up the whole household even when I have turned the one I am wearing off! How can I get the one I am not wearing to not sound the alarm?! It is driving my husband mad … please help!

[Re-Titled by Moderator]