First, don't worry about damaging your amp. Even with the load impedance cut in half, unless you really drive up the volume, you're not going to hurt anything.

What you are missing here is the frequency dependance of the crossover.

Think about your two drivers (woofer and tweeter) in parallel. First, the woofer impedance is going to be quite high at tweeter frequencies. As such, the 8Ω impedance of the tweeter in parallel with the woofer (at high frequencies) means that the parallel combination is fairly close to just the impedance of the tweeter. But, bass frequencies can really hurt a tweeter due to it's "relatively" lighter construction. So what is generally done is the tweeter is put in series with a capacitor. The capacitor blocks the lower frequencies from the getting through and tweeter only sees the high frequencies. In many inexpensive 2-way speakers, the entire crossover is just this capacitor. This is the "poor man's" solution.

Now, in reality, things are not quite that simple. The impedance of the driver works in conjunction with the crossover components and forms a frequency selective filter. So the series capacitor and the tweeter form a high pass filter. The impedance is very high at frequencies below its lower corner frequency, so at these frequencies, the amp only "sees" the woofer. But at high frequencies the amps sees the two in parallel. And even though the woofer impedance get pretty high at higher frequencies (and it's audio response is very low) you can still do better.

The solution in this case is to now put an inductor in series with the woofer. Then the impedance of the inductor and the woofer form a low pass filter, and the woofer only sees low frequencies. Together (if designed correctly) they form a matched pair which has reasonable impedance across the entire frequency band. This is an example of a basic single pole, 2 way crossover.

There are lots of variations depending on what you are attempting to achieve, but the basic idea remains the same. Take a look at this website (

http://www.audioholics.com/loudspeaker-design/crossover) and see if it answers your questions. If not, come back and ask some more.