Two loudspeakers serially ?

Exactly the same as connecting two drivers without crossovers in series, see: speakers in series.

The only difference is that the impedance seen before the crossover has a different characteristic than a driver without a crossover.

Connecting in series won't necessarily do anything 'bad', but it likely will change the frequency response for the worse. Also, connecting two different multi-way speakers in parallel isn't advised either, because the phase response of each multi-way speaker is unlikely to match so there will be partial cancellation.
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