+1 on the two strips in series. Def easiest solution if it is feasible in your project.
The alternative is to determine the total resistance of your one LED strip and then put a matching resistance in series with the strip. If your strip is 4ft long and uses 80mA per foot at 12V, then 4 ft will use 320mA. R = V/I = 12 / .320 = 37.5ohms, which is orange purple black (and probably brown or gold for the tolerance ring, 1% or 5% tolerance respectively). It will have 320mA going through it at 12v so 3.84W is the power handling it has to support. I'd opt to get 5W for safety. Lastly, if you can't find the exact match, try multiplying the ohms by 2 and halving power handling capability and get a pair of resistors (you'd wire them in parallel to each other and then the pair in series with the LED strip).
Lastly, why not use a voltage regulator instead?
RadioShack 12V Fixed-Voltage Regulator 7812 : Voltage Regulators | RadioShack.com Cheap, simple, effective. Get a couple micro farad capacitors and you're off and running with 12VDC from anything up to 30something VDC input and up to 1A of output current handling. Lookup a 7812 datasheet for other circuit configs if you want variable output voltage, etc.
