Does it mean we need to match the sensitivity while choosing the drivers?
Another doubt is, if each driver has different impedance, what could the nominal impedance of the speaker as a whole?
The sensitivity should be matched. If thats not possible, a tweeter with sensitivity higher than woofer with LPAD Attenuator should be used to match the sens. It is not advisable to use attenuator with woofer.
The second question is tricky. If the impedance is not matching, the resultant final impedance will depend upon the cross over point. It is difficult to calculate. Needs to be measured but i dont know how. However with simple logic I can say that if impedance is not matching, the crossover point should chosen such that the driver with lower impedance covers narrower band of frequency than the other one so that the final impedance remains on as higher side as possible.
If you choose 8 ohm and 4 ohm speakers and wire in parallel without xo, the resultant imped will be around 2.5. If you use crossover, it will be definitely higher than 2.5 but how high will depend upon the crossover point.
If woofer is 8 and tweet is 4, keep xo as high as possible(depends upon how high up the woofer can play) so that final impedance is as high as possible.
If woofer is 4 ohm and tweeter is 8 ohm,keep xo as low as possible (depends on how low the tweet can come) so as to keep resultant impedance as high as possible.
If your amp is robustly built, you dont have to bother much about impedance.
I had connected 8 ohm tweeter with 8khz high pass filter, 8 ohm fulls ranger without cross over, 8 ohm woofer with 250hz low pass filters in parallel to my marantz pm6002 and the amp had no issues at all. Did not even become warm,at least for the volume level that I use. So i dont know what was the final impedance but marantz could take it easily.
I hope this helps to some extent without causing much confusion.