Series circuitry with passive crossovers

Two similar (same impedence) speakers wired in parallel to a passive Xover will DOUBLE the frequency X point since the impedence is cut in half.  Two speakers wired in series will cut the X point in half.

Yes, it is possible to overload the amplifier channel by changing the impedence load a passive cross is designed for.  This would happen if you, for example, add a second woofer into the low pass section of a 2-way network but not to the high pass section.  The low-pass X-point doubles, up from say 2400 Hz to 4800 Hz.  Besides the fact that some woofers cannot accurately reproduct 4800 Hz, the high pass section will still function at the original point of 2400 Hz.  Thus, between 2400 Hz and 4800 Hz all three speakers are connected in parallel to the amplifier.  This could easily drop the net impedence load down too far and overload the amplifier.  Not to mention the terrible sonic effects you will get.

So I recomend that if you need to add speakers to a passive crossover filter network, redesign it for the new impedence loads and build a new one.