Passive speakers are naturally balanced, the necessary conditions being (a) stray currents induced in the cable see the same impedance on both wires, therefore producing identical voltage deviations (common-mode noise), and (b) the sound produced by the speaker is a function of the voltage difference between the two wires.
If induced noise is a problem in such a setup, the actual improvement to be made would be to use twisted pair or shielded cable, rather than bare parallel wires. This helps eliminate differential-mode noise due to near-field EMI which parallel wires are susceptible to.