Even if they cables are the same type, they really should be the same length. This is so that the loading on the two amplifier channels is the same.
As for mixing and matching: If you are on a budget, then using one for the treble and the other for the bass can be synergistic depending on the system.
As for mixing and matching: If you are on a budget, then using one for the treble and the other for the bass can be synergistic depending on the system.