Lacee, while there is no harm in using different material in the separate legs of the speaker cables, there is no real 'rationale' for doing so, since the current is alternating and not concontinuously 'flowing' in one direction as the terms 'lead' and 'return' imply. The current goes 'back and forth', so it doesn't make any difference what material is used in the separate legs. Which also explains why there is no 'direction' of the signal.
Bob P.
Bob P.