the question is a bit vague, but, assuming you realize you’d get the same stream on both, so it would still be mono, yes.
for the small prints now… you need to be sure that your tandem of speakers is wired in such a way that the total impedance is adequate for the amp on- oard Speaker pHAT.
see the following topic for a discussion on the matter - it is about the Picade HAT but I am 99% sure that the amp part is the same, the discussion is generic enough to be relevant in any case:
http://forums.pimoroni.com/t/picade-hat-volume-control/3720/9