You do not have to have two sets of preamp outputs to connect two amplifiers....it just eliminates the need for using Y-splitters to carry a single output signal to two destinations.
But you can run 2, 3, or more amps off of the same set of preamp outputs. Just be aware that the more you split the signal the lower the preamp output voltage being sent to each amp is and will, as a result, raise the potential noise floor.