Behringer digital crossovers are very popular these days for bi-amping. One of these units would go between your preamp and power amps. Some of these crossovers have XLR connections, which will work fine if you have balanced equipment. If not you will need adapters or wires that are XLR on one end and RCA on the other. You need only disconnect the internal wires to the speakers from the built in crossovers, and in turn run wires directly from the amp outputs to the individual speakers. You can re-attach the old crossover wires later, if you want to restore the speaker to it's original condition.
If you decide to horizontal bi-amp, and use the amp that you have for the top, then you will not have to get the exact same amp for the bass drivers.
If you decide to horizontal bi-amp, and use the amp that you have for the top, then you will not have to get the exact same amp for the bass drivers.