Yes to your first question each SIM can be a different carrier/ plan.
No on the second. The watch has a single eSIM which gets registered with a carrier and then they use number sharing/forwarding from whichever phone you have registered with that carrier.
Now any functions that work on the watch that are not cellular specific — as in via wifi or Bluetooth — will work on your watch as relayed from the phone it’s linked with, regardless of which cellular network it may be getting it’s data from. So that will be handy for travel situations.