The double/half frequency recommendation for xover use is generally applied to a xover with -24dB slope. Specifically in the case of many prepros, the low-pass gets a -24dB slope, but the high-pass only gets -12dB slope because they expect your speakers to roll-off with a -12dB slope thus summing to -24dB slope for the high-pass.

Typically the only speakers that meet this -12dB roll-off spec are THX speakers @80Hz.

Sealed or acoustic suspension speakers also typically have a -12dB roll-off, but typically not at 80Hz, making them a difficult match with the prepro.

On the other hand many ported speakers have -18dB, -24dB or even -36dB low frequency roll-offs that certainly don't combine with the prepros -12dB slope to produce a smooth symmetrical xover between the high-pass and the low-pass.

Just some of the gotchas in bass management.