Active Crossover Recommendations

Rhyno, I assume you;re referring to a passive line level low-pass? If so, 1st order Butterworth is C= 1/2pi(F)(Ramp)(1+a). F= xover frequency, Ramp is the amp's impedance, a is the relation between amps' gain.
I.e. if a=1, the amps to be used have identical gain. If not, give a the appropriate value to balance the gain. For a much better take on this, check out Art Ludwig's site (sorry don't have it handy).

the cap would of course go in series w/ the high freq amp, while the low freq amp (the sub) you could simply rely on its own internal crossover to filter out higher freq info

For hi pass, cap in series, correct.

Matching the two xovers, however, may take some experimentation: if the sub's xover is not 1st order (6dB/octave) electrical -- which it probably isn't -- you may just get away with playing w/ the low pass frequency. This worked for me once, where the powered woofers part of the speaker system was detachable from the rest of the speaker; the cut off for the woof (12dB/electrical) was set ~1 octave lower than the main; then we used placement of powered woofs to compensate for the phase differences between the two.