It depends on how you connect the two speakers to the amp.  If you
wire them in series, the overall impedence will be twice that of a
single speaker.  If you wire the speakers in parallel, the overall
impedence will be half that of a single speaker.  You need to take
into account the impedence of each speaker, and the recommended
impedence range for the amp to make sure that you're not overstressing
the amp or wasting too much of the amp's power capability.

Scott Gardner