Use a Y-splitter in the sub-out jack of the receiver to hook up two subs.
If you go with the ED speakers, you will blow your entire budget on speakers and receiver, and you will be left with no bass. I don't think it would sound very good. If you intend to buy a sub later, that is alright, but you will need a hell of a sub to keep up with those mains. It won't be cheap, or small, or light-weight. You would want a pair or ED A7-350s, or Hsu VTF15hs, or SVS PB12 Pluses, or something like that. In the end though, you would have something that absolutely pounds. However I think you might regret it because you would never even use half the system's capability. And if you actually did use half the system's capability, it wouldn't be long before you suffered permanent hearing damage.