You can pair multiple watches on the same iPhone, but only one will be active at a time
Have a look at 'Pair more than one Apple Watch, paragraph in this page:
Got my second watch today. Have two different wake up alarms. tried it out, watches work, but my iPhone tried to wake me up twice. Called apple support and they told me it would work for the watch I had on at the time. Will see what tomorrow am brings and I’ll report back.