The woofer has a lowpass in series with it, the tweeter has a highpass in series with it, and those 2 "assemblies" are wired in parallel. Now think about how the circuit behaves at various frequencies, let's start at DC.

At DC the woofer's lowpass has an impedance of 0 and the woofer has an impedance of its Re (around 3.3ohm), since they're wired in series the woofer circuit has an impedance of 3.3ohm. At DC the tweeter's highpass has an infinite impedance and the tweeter has an impedance of its Re, since they're wired in series, the tweeter circuit has an infinite impedance. Now as far as the amp is concerned, you have a 3.3ohm impedance wired in parallel with an infinite impedance, and the result is 3.3ohm.

At very high frequencies you have the opposite, the tweeter's highpass has an impedance of 0 and the tweeter has an impedance around 4 or 8ohm depending on whether it's a 4 or 8ohm tweet. On the other hand the woofer's lowpass has an infinite impedance, the woofer has something like a 10-20ohm impedance (it rises at high frequencies due to the inherent inductance), and so now the woofer's circuit has an infinite impedance and the tweeter's circuit has a 4 or 8ohm impedance, wired in parallel you just end up with the tweeter.

At frequencies around the middle you get a mix, up until the crossover frequency the HP and LP filters will be very close to 0 and infinity respectively (or vice versa depending on which side of the xover freq you're at). When you get close to the xover frequency, the filters start to switch, the one that was 0 starts to rise towards infinity, and the one that was very very high starts to fall towards 0. At the crossover frequency, if you have say 4th order LR filters, each filter's impedance will be exactly equal to the impedance of the speaker that it's controlling. So the woofer's lowpass filter will be equal to 4ohm, and the tweeter's highpass filter will be equal to the tweeter's impedance. Since the filter is in series with the speaker, the woofer's effective impedance will be double, and the tweeter's effective impedance will be double. When you wire these two in parallel, the result will be right around the nominal impedance of either of them.

If you have a 4ohm woofer and a 4ohm tweeter, the final impedance should be right around 4ohm throughout the entire frequency spectrum.

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