You need matching sensitivy (dB) for booth fullrange speaker and tweeter, also look out to check if the senstivty curve is flat.
Sensitvity means if you put in 1w the ammount of volume you get out, same sensitivy means speaker gives same volume with same power input.
Is mostly impossible to get very good mathcing without the use of cross over on both woofer and tweeter when crossing above 6khz.
A simple 1st order on both high and low should work.
If the senstivity is not same as woofer but higher, you can use high power resitors and cut down the voltage at tweeter. By using a resitor divider and calculating values that keep the total impeance at 8 ohms.
Also keep in mind that half the voltage is not half the power to woofer. So you need to convert the calculated reduction in voltage from divider network to power.
http://www.sengpielaudio.com/calculatorVoltagePower.htm