Today it depends, on the level of systems management available by the Computers.What is the purpose of 2-seat J-10s? Is it only for training or does it have any tactical benefits?
Back in the day it was more critical. In 3rd gen fighters and early 4th gen it was critical as the electric systems, weapons and Radar were more attention demanding. So the pilot needed an extra set of eyes, hands and brains to operate those.
Those fighters often omitted any ability for the back seater to fly the plane.
The Russians even debated using the back seater station for operations command in Mig25 where the back seater would have been a senior officer who would have commanded forces in operation like a Super sonic awacs. Nothing ever came of it though.
In some 4th gens the back seater is more specialized to air to ground. Navigating, operating jammer pods, and preparing for ground attack well the pilot can focus on flying the bird. In later versions the back seater can even fly the plane.