When you wire speakers in parallel, the equation isn't as simple as with series
This is what it looks like when you wire up two speakers in parallel:
This is what it would look like if you were to wire up four speakers in parallel:
The equation for this is as follows:
1 / Ztotal = 1 / Zspeaker a + 1 / Zspeaker b + 1 / Zspeaker c + ... + 1 / Zspeaker z
For 2 speakers (namely a and b) the equation simplifies to:
Ztotal = (Zspeaker a x Zspeaker b) / (Zspeaker a + Zspeaker b)
Note: multiplication on top and addition on the bottom of the fraction!