Why would a dual voice coil only work in parallel but not series?

As I recall. the Gallo woofer was a dual coil affair. It was designed to be driven by the owner's amp AND a Gallo amp. I guess you could use either or both.

But it was a mono amp.

I'd imagine the Gallo amp provided the current and the owner's amp (OA) could provide the signal. Assuming the OA was able to dominate the waveform (just small amplitude changes) then you'd get the sound of the OA with the muscle of the Gallo amp.
 
No, the Gallo woofer is not like that, it a passive woofer... the amps are all separate.

https://hometheaterhifi.com/reviews...ny-gallo-acoustics-reference-35-loudspeakers/

"...The dual voice coils are configured the same as in the previous 3.1. One voice coil is connected to the main binding posts through a ~125 Hz low pass crossover. The other voice coil is connected to a second pair of binding posts on the speaker with no crossover present..."