As soon as you add a second sub you have more challenges to deal with in blending not only with the speakers but also with each sub. Pairing the same sub usually makes it easier to blend the subs together (same phase response and tuning frequency) and blend both with the speakers. Purists would say run the same subs but if you are careful good results can also be had with different subs as long as the tuning points are reasonably close or placement is such that phase anomalies are minimized. Aesthetics are not as good with different subs which may not matter.
Can't answer the eD sub question without more info about which sub you have in mind.
