The advantages of vertical bi-amping are that each amps power supply only has to drive the bass on one speaker. You will experience greater volume and dynamics, with a more effortless musical presentation. And used Aleph amps are reasonably priced.
On the other hand horizontal bi-amping, like one reply states, still requires the that the amp driving both channels of low frequencies still puts a greater demand on that amps power supply. This defeats the advantaqges. Anyway, for sonic balance both amps should be identical.