it not about training and conversion, 2 seater have some advantages over 1 seater, in single seater MULTIROLE jets pilot can distract/overload his capability to do both missions air to air/ air to ground, whereas in 2 seater separate pilots for separate for separate duty minimizing pilot distraction/ pilot overloading for both pilots, as you look to the history of fighter jet developments in early 50's, there were a separate jets or version of same jets doing different missions, as microelectronics industries improves their products in 70's,80's and 90's there is no needs of separate air to air/ air to ground jets, but 2 seater has above mention advantages over single seater
@GumNaam