Hey there,
The Isle of Siptah DLC is available on Single-Player, Co-Op, Official servers which specifically host the Isle of Siptah and Dedicated Servers for all players who have purchased both the Isle of Siptah DLC and the base game.
You can only run one map at a time, but you can choose to switch between them without losing progress (seperate databases/saves, seperate characters - no character transfers).
Dedicated server owners who want to host the Isle of Siptah map should read this thread: