Elite Specialization
Yes, you will only be able to use one at a time.
Some must fight so that all may be free.
They already answered this when HOT came out. You can only load one Espec at a time, and is uniformly restricted to the 3rd trait line slot to make them mutually exclusive.
Especs also mechanically restricts the special weapon associated with it; so changing out of an Espec causes you to lose access to it until loaded back in again.