Can you have 2 phone numbers on 1 Apple Watch

Yes. If you have an iPhone using dual sim, you can add both plans to the Apple Watch as long as both carriers support the Apple Watch and the plan is legible for it.

click here ➜ Use Dual SIM with Apple Watch GPS + Cellular models - Apple Support

You cannot use 2 iPhones with the same Apple Watch. And you cannot add 2 plans from 2 different iPhones to one Apple Watch.