Some 1,000 artists and arts organizations are now working in the Dumbo section of Brooklyn, courtesy of the developers David Walentas and his son, Jed, partners in Two Trees Management. Operating on the principle that cultural ferment makes a neighborhood hot, Two Trees has offered creative people rents that they cannot refuse. “It adds value to any neighborhood,” David Walentas said in an interview at a conference table in his unflashy Dumbo office. “It’s like good architecture. Good architecture is cheap and adds value. People will pay a premium for it.