(b)where the concurrence of other persons is necessary for validating the title, and they are bound to concur at the request of the vendor or lessor, the purchaser or lessee may compel him to procure such concurrence, and when a conveyance by other persons is necessary to validate the title and they are bound to convey at the request of the vendor or lessor, the purchaser or lessee may compel him to procure such conveyance;