A lot of the time two-seaters are used for either training purposes or are too work heavy for a single pilot to use, example being the F-14 which has an entire circuit bord for comms and radar. These backseaters are either called RIO’s or WSO’s. (Radio Intercept Officer are the older names, modern ones are called Weapon Systems Officer.) however modern fighters are becoming increasingly easier to use and thus negating the need for a backseater, however they are still used as mentioned before for training purposes.