Felipe II's royal decree was based entirely on  the idea being that Madrid, smack in the center of Iberia, would promote centralized authority over regional power bases in a newly unified Spain.
Felipe had a royal decree about Madrid.
Other benefits, such as a single mobilization, a single construction manager, and sharing of large construction equipment for the two projects exist, but they are not expected to make a significant difference due to the difference in size between the portions of the combined FGD and ACI project.