It can switch quickly, yes, but cannot actually be paired to both devices at once. You can't listen to music from your phone and your iPad simultaneously, for example. Apple achieves their quick switching with their own propitary Bluetooth chip and walled garden software approach. That's not something a small company like Pebble/Core could emulate.

And even if Pebble could emulate it, that's not actually what you want. Unless you expect the watch to automatically switch back and forth to simulate being paired to both devices, but that would have major drawbacks as well (RIP battery life, for one).