It doesn’t take any special cable to connect for passive biamping. Just use another set of cables like the first. I could tell you that bundled biamp cables puts the hf and lf magnetic fields too close in proximity. Some believers claim separating the hf and lf magnetic fields is one goal of biamping. I am not a believer in magic cables. Amplifiers are voltage sources. Separate speaker sections impedance, current, and power are the same whether driven by one amplifier or separate amplifiers. Passive biamping doesn’t increase amplifier power, it divides the speaker impedance, current, and power between separate amplifiers. Passive biamping retains the passive crossover. The passive crossover impedance separates the high and low frequency current.