Faced with these two pairs of Mangekyō Sharingan, both practically bug-level, Minagawa Uchiha wanted the Dojutsu within both. However, according to information from the Original Work, fusing one pair of Mangekyō Sharingan could elevate his own Mangekyō Sharingan to the perfect Eternal Mangekyo Sharingan. Whether additional Mangekyō could be fused afterward remained highly uncertain.
This fusion was more like compensating for a flaw. Fusing one pair of Mangekyō Sharingan could be seen as perfectly making up for that flaw and reaching a complete state. Fusing a second pair after that would be like attaching another flaw onto something already perfect. It would most likely fail or result in some other accident.
Therefore, to be safe, Minagawa Uchiha would only choose one of the two. Compared to Kotoamatsukami, Kamui was clearly a better fit for him, both in terms of ability and function.
There was no other reason. Obito Uchiha's eyes were a complementary pair. Not only could they unleash a Space-Time Dōjutsu, but they also possessed a Kamui Dimension. Minagawa Uchiha guessed that after fusing those eyes, the Kamui Dimension might merge into his Dream Space. Perhaps, within that illusory dream world, he could possess an eternal space that belonged solely to him!
This fused space would likely inherit Minagawa Uchiha's absolute control over everything within the Dream Space, while also gaining Kamui's ability to open and close the space anytime and anywhere at little cost. Furthermore, when absorbing other external objects, he would no longer need to consume vast amounts of chakra to first turn the real into the unreal as before. Any Kekkei Genkai he wanted to fuse afterward could be completed directly within this fused space.
Perhaps this space, formed by combining the traits of reality and illusion, was no longer a space in the ordinary sense.