Descriptions of alchemical processes often describe the reactions of sulfur and mercury. The Red King is sulfur -- the active, volatile and fiery principle -- while the White Queen is mercury -- the material, passive, fixed principle. Mercury has substance, but it has no definitive form on its own. It needs an active principle to shape it.
the divine hermaphrodite—a reconciliation of spirit and matter, a being of both male and female qualities as indicated by the two heads within a single body.