It's quite natural for a plane to loose some agility (especially a small plane) when you add in the extra weight of a second pilot, ejection seat, life support systems and instrumentation/controls. That is just the bare minimum if you want to put in a second pilot. If you want other goods to take advantage of the second person, like a more sophisticated ground attack capability, or more advanced EW etc, well, that's even more weight.
And of course, since you are not making the plane proportionally bigger to make room for the second pilot (again, because of weight), you are forced to take stuff out instead. The easiest thing to cut if you want to also add in more avionics would be fuel. That means less endurance, and shorter use of afterburners. So a further reduction in air combat potential.
But that is hardly a new or unique problem, which is why most air forces rely on their single seaters for air combat, and use their twin seaters for ground attack and training.