This strategy of killing two birds with one stone may seem casually easy, but Kexiao didn't take it lightly at all, especially when dealing with issues on the southern front. After all, Amelia did have some abilities, but at best, she was only a leap in thinking beyond the ordinary. Once you see through her next plans entirely, the threat she poses is hardly concerning.
On the other hand, the problem of the Twin Stars was indeed thorny. Kexiao also feared that he might not manage everything well. The simple truth of choosing the lesser of two evils or the heavier of two benefits was not beyond his understanding. Feng Yi might appear to have only these troops, yet the threat he posed was far greater than that of Amelia. You never knew when his small force would begin to snowball, bigger and bigger like a rolling snowball. Feng Yi was the type of existence that needed to be monitored at all times. Once you gave him room to grow, he became far more fearsome than Amelia.