JackPhoenix's answer is, of course, correct. Technically speaking, shield is neither Light nor Heavy nor in between, because those are weapon properties and shields are not used as weapons, by default.

Light property is actually only relevant to Two Weapon Fighting and avoiding possible penalties for that style, and has nothing to physical weight. Thus there is nothing special about Mace + Shield fighting, since it is not TWF.

Now where this might sneak in is through the Improvised Weapon rules, if the DM rules "this chair leg is just like a Club which is Light" or "this blacksmith's hammer is just like a Light Hammer which is Light". I do not see a shield has getting leeway here.

But since you brought this up, the idea of a subclass or Feat for fighting with two Shields brings a smile to my face...