More subs = smoother bass throughout the entire listening area.
Sure, you can crank them both up if wanted, but the real benefit of dual (or more) subs is getting better response in the room from seat to seat. You may find that with one sub, your seat is in a null and you barely notice the bass, or it is lacking quite a bit, but 4 feet to your left it is really good (or maybe even too much). Putting a second sub in at a different location (a whole other conversation on placement) and if done right, it can feed into that null and lift up the levels there, while pulling down some of the over the top bass at another location.
Now if that means two EP500s, or a EP600 now and something else down the road is up to you.
