Letts jump directly to the deal: If you buy a Lumia 950 XL from the Microsoft Store in the US, you will get a Lumia 950 free. And there is no locked or carrier mumbo-jumbo here. The phones are totally unlocked.
The Microsoft Store is selling the 950 XL for $649 while the price of the Lumia 950 is $549. i.e. You buy two phones -- Lumia 950 XL and the Lumia 950 -- at the total cost of $649.
P.S : This deal is available only in the U.S. as of now.