When he awakened his Mangekyō Sharingan, he had already realized the potential of the Susanoo Lamp.
After he gained control of Fugaku Uchiha's pair of Mangekyō Sharingan, he experienced the Susanoo Lamp's potential firsthand.
Controlling a pair of Mangekyō didn't directly double his Dojutsu, but the boost to his Dojutsu and the additional techniques gave him a great deal of leeway in later battles.
However, after he fused a pair of Eternal Mangekyō Sharingan...
He found that using the Susanoo Lamp to control a pair of Mangekyō Sharingan provided only a very limited boost to his Dojutsu.
And as the power of his own Eternal Mangekyō Sharingan grew stronger...
The boost he gained from controlling an ordinary pair of Mangekyō became increasingly feeble, leaving him with little more than the benefit of their techniques.
At first...
He thought it was because there was too great a gap between the power of the two pairs of eyes. But when he had no choice but to control that Byakugan on the moon, he discovered that the boost to his Dojutsu from controlling the Byakugan was actually greater than what he got from controlling a pair of Mangekyō Sharingan.
And by quite a lot!
Although that Byakugan was no ordinary eye—it came from the Otsutsuki Branch Family on the moon and was of exceptionally high purity—in terms of Dojutsu, it was clearly inferior to a pair of Mangekyō Sharingan.
Yet when controlled with the Susanoo Lamp, the boost to his Dojutsu from the Byakugan was greater than the boost from the Mangekyō Sharingan.
This left Shinji puzzled.
After many more attempts and experiments...
He began to sense a secret behind the Susanoo Lamp: controlling different Dōjutsu Kekkei Genkai granted a far greater increase in Dojutsu than controlling the same kind.
This discovery led