As @erik_squires noted, with most speakers the stock/internal crossover is not simply a set of filters that divides the entire input signal into two separate frequency bands, one for the woofer the other for the tweeter (or tweeter/midrange driver). The internal crossover in most loudspeakers also includes components (resistors, capacitors, etc.) that tailor the response of the drivers. External analog crossovers provide only "textbook" filters (1st/2nd/3rd/4th-order, which creates a 6, 12, 18, or 24 dB roll-off at the top or bottom of a driver's response). A textbook crossover can NOT provide driver compensation networks. It couldn't, as all loudspeakers have different ones for their specific drivers used in the way the speaker designer employs the drivers.
However, there ARE some loudspeakers which have internal crossover which do not include driver compensation networks. Magnepan, for instance. In Magnepan's pre-.7 iterations of their loudspeakers, the drivers were run in parallel, so they could be separated from one another for bi-amping. When Magnepan introduced the .7 version of each model (1.7, 3.7, 20.7) they went to a series configuration, which means the drivers can not be separated for bi-amping (at least not without doing "surgery").