Hi @ Louise Howarth,
You don’t need to have a SIM card in both.
It may depend on what you want to do.
The watch can function as a “standalone phone” if it has a SIM card installed, '''BUT''' ''it only works on 2G mobile phone networks.'' If your preferred service provider doesn’t have a 2G network in your area (or there are none at all) it won’t work as a standalone phone or be able to connect to the internet etc via the mobile phone network.
If your mobile phone is connected to a 3G (or later generation) network then the watch can be used to make/receive calls via a Bluetooth connection with your mobile phone’s network connection. The watch doesn’t need to have a SIM card.
Here’s a link to the [https://www.slideshare.net/secret/Gr0ZtuQH43RjqX|user manual] which may be of further help.