When david o'doherty and katherine parkinson are both on the davids team when is the first broadcast?
CREATE TABLE "table1_23575917_6" (
  "episode" text,
  "first_broadcast" text,
  "davids_team" text,
  "lees_team" text,
  "scores" text
);
SELECT "first_broadcast" FROM "table1_23575917_6" WHERE "davids_team"='David O''Doherty and Katherine Parkinson';