In short, yes, it will be louder.
In the simplest case, if you were able to duplicate the exact signal everywhere in space, you would actually get 4 times the intensity - sound waves add linearly, but intensity adds quadratically. For two uncorrelated sources (if you played different white noise signals through each speaker) you would only get a factor of 2. In the real world, where not everything is exactly in phase everywhere in space will be something in between. (I'll add a sketch of the math later).
But you probably know, it doesn't sound 4 times louder. That's because your ears hear logarithmically - exponential increases in intensity are heard as linear increases in volume. Volume is typically measured in dB (a logarithmic scale), here is a reference that can calculate the dB of coherent and incoherent sources.