It's not as much the wattage, as it is the impedence you get from wiring in different ways. If you had a sub with a single voice coil, you can't wire in series or parallel. It is only used with more than one subwoofer, or with subs' that have 2 or more voice coils.
When you wire two voice coils together in series, you add the impedences together, (example: two 4 ohm voice coils wired in series would present a 8 ohm load) It's the same as if you wired two 9 volt batteries in series, you'd have 18 volts.
Parallel: When you wire two voice coils in parallel, you take divide the value of one voice coil by two. (example, if you wired two 4 ohm voice coils in parallel, you would get a 2 ohm load.)
ok, but sum1 told me that 2 svc subs wired in parallel only get half the watts to each sub, but i thought both subs got max power wired in parallel and split the watts in series. whats right? (thanks for helping me out man)