The problem is that your speaker would create a pathway AKA circuit that the power from amp one would travel to another. I am embarrassed to say I have done this experiment. The only way you might be able to biAmp is to use a really carefully designed crossover outside the speaker that would override your speaker's one crossover (wich should be disconnected). It is a major undertaking some of the other suggestions make sense e.g. bridging etc.
