If you are talking about raw drivers the impedance as seen by the amp is
related to the x-over not the drivers in particular.
The issus as I see it is the sensitivity twixt the two drivers.
You'll need a "notch" fitler to lower the tweeters sensitivity
to better match the woofer section's sensitivity.
Add up resistance in series and half the lowest for parallel
(althought that's a little too general)
So the point is your total ohm load is not sole'ly a deriviative
of the driver. There is addtitional resistance - inductance - capaticence
from the cross over....
There is a TON of info out there
http://www.passivecrossovers.com/
Some really good reading is this little booklet
http://www.pac-audio.com/crossover/crossovermain.htm
I can go on and on...
Calculators....
http://www.carstereo.com/help/Articles.cfm?id=10
Good Luck!