Using 2 sims from different providers for Apple Watch

I have an Apple Watch (GPS + Cellular). My iPhone is on Tesco mobile and they DO NOT support cellular plans for Apple Watches. Can I use the GPS from my phone as default, and get a PAYG plan from another provider (eg O2 who provide to Tesco) for the watch, to use on relatively rare occasions where the phone may not be nearby, such as texting from the sea/in a pool etc, and if so would all the other apps work such as whatsapp with the phone provider on my other number when in range - so it just switched when out of range?