The new plan is aimed at couples and roommates, providing a separate Spotify Premium account for each person so they can enjoy their own music. Both users on the plan must live at the same address, and Spotify requires a confirmation of address during the sign-up process. Spotify Premium for Family (up to six people sharing the same plan) also requires users to live at the same address, but many people still use it to share accounts from afar with their friends, so Spotify appears to be attempting to curb this behavior with Premium Duo.
Premium Duo comes with an exclusive playlist called "Duo Mix," which Spotify regularly updates based on music that each user on the plan listens to. On mobile devices, Duo Mix will also have two alternate versions that users can switch to with just a tap: "Calm" for more mellow tracks and "Upbeat" for songs with a faster tempo. Additionally, the two members on Premium Duo will be able to create playlists and share them with one another thanks to a Shared Playlists feature.
The bill for the plan will be sent to the person who sets up and purchases the Premium Duo account. In terms of pricing, it appears that the new plan sits somewhere between a normal Premium membership ($9.99) and a Premium for Family membership ($14.99).
Head to Spotify's website for more information on Premium Duo.