Stilton, this article indicates the USAF intends to normally operate the EX with single crew, but the aircraft will retain the second seat. Imagine that will allow some flexibility for training and high workload flights where 2 brains helps, and future systems.
I believe all the advanced F-15's produced for other air arms recently are twin seat, so think this was the most expedient add on order for the USAF.
https://www.flightglobal.com/fixed-w...142864.article