Okay, let's use a simple two-way filter: a series inductor on the woofer and a series cap on the tweeter, with these two sections paralleled. Thus, we can look at the two speakers' high or low sections independently, as just two tweeters or just two woofers.

As you know. the effective crossover frequency depends on the driver impedances, so when you series the two caps and two tweeters (or the two inductors and the two woofers), you double the effective driver impedance, so the overall crossover frequency remains.

The point is that two identical loads in series (or parallel, for that matter) will each behave as it would alone, with the exception of requiring twice the voltage (for series) or twice the current (for parallel) from the driving (powering) device.

Now, dissimilar loads, which each vary impedance with changing frequency differently, will, indeed, behave in undesireable ways when in series, because the voltage across each load will vary as the impedance imbalance shifts back and forth as you traverse the frequency range.