“Bâtiment” is masculine singular.
The first blank must be filled with an interrogative adjective because “bâtiment” is a noun that follows immediately. Quel is the correct masculine singular adjective. Quel bâtiment dessines-tu ? (Which building are you drawing?)
In the second part, “lequel” is used because the noun is not repeated — we’re referring back to “bâtiment.” Lequel dessine Pierre ? (Which one is Pierre drawing?)
Final sentence: Quel bâtiment dessines-tu et lequel dessine Pierre ?