That is a series crossover, because the coil is shared with both voice coils. Most crossovers are parallel where each driver has separate discreet crossover filter. Series is said to have some sonic advantage, ymmv, idk
http://sound.whsites.net/parallel-series.htmIn a parallel crossover, the coil size would also change with load impedance, but here you're suggesting it does not. dunno
For standard textbook parallel filter, if the load impedance is halved, the size of the caps doubles and the coil is halved. There is no online calculator for series crossovers, but filter theory still applies. It seems you are correct to assume it could be the 8 ohm crossover.
Try it and see. You should be able to build this simple design in stereo for about $40 shipped.
It is a very simple design, you might find that some additional parts could smooth out the frequency response.
Good luck, and thanks for posting on AudioNervosa.com!
Rich