You should be able to upgrade using the device payment plan while you have unlimited data. That became a "perk" when the unlimited data price increased. You can also opt to pay full retail in one lump sum or look at 3rd party retailers to purchase one (such as eBay or other sites like that).
The unlimited data plan is not shareable. In order to add the Gear S2, it would need it's own data plan. You can't add it to the unlimited plan.
Unlimited has never been shareable - that's why you pay extra to add the hotspot if yo have an unlimited plan, but not when you have a shared data plan.