Two ways to wire two speakers together....
parallel:
+.......W.I.R.E......+.........W.I.R.E... +
Spk1 Spk2 Jack
-.......W.I.R.E......-..........W.I.R.E...-
For parallel, use the following formula....
Total Ohms =1/[(1/R1)+(1/R2)]. So if you have two 8 ohm speakers, it would be (1/8)+(1/8), which is 1/4, then take 1/(1/4), which is 4. If you have 16 ohm speakers, using the same formula you get 8 ohms.
If it istn't parallel, its series (which I won't draw). In series, you add the ohms of the speakers. So if you have two 8 ohm speakers, then total is 16 ohms. If you have two 16 ohm speakers, then total is 32 ohms (highly unlikely). I wouldn't think a bass amp would be wired for 16 or 32 ohms, but I've definetly been wrong before.