Can't Wear Two Sets
Can't wear two sets of armor according to the rules of magical items.
A character can't normally wear more than one pair of footwear, one pair of gloves or gauntlets, one pair of bracers, one suit of armor, one item of headwear, and one cloak.
Would Only get AC from One if you Could
Sage Advice makes clear you can only use one formula for calculating your AC at a time.
These methods—along with any others that give you a formula for calculating your AC—are mutually exclusive; you
can benefit from only one at a time. I
The Molten Bronze Armor is one way.
Breastplate 14 + Dex modifier (max 2)
Half Plate 15 + Dex modifier (max 2) -- --
Normal armor is another, and Efreeti Chain +3 is Heavy Chain so it would use this formula:
Chain mail 16 [+3 from Efreeti Chain]
So, Molten Bronze Armor Plate would be 18 and Efreeti Chain would be 16 + 3 = 19. You can choose to take the 18 or the 19, but Rules as Written, you can't do 18+3.
Arcane Armor Only Works on One Anyway
If you could wear two sets, you could turn either into Arcane Armor, but not both. The lines here clearly assume one set of armor, because that is what the rules dictate.
As an action, you can turn a suit of armor you are wearing into Arcane Armor, provided you have smith’s tools in hand. ... The armor continues to be Arcane Armor until you don another suit of armor or you die.
Assume you Could... Benifit?
For the sake of argument and completeness, let's assume your DM allows it, despite being against rules as written.
- If the armor normally has a Strength requirement, the arcane armor lacks this requirement for you.
Shouldn't be a big deal. Chain doesn't have strength requirement. You'd just make the Bronze Skin the Arcane Armor because it has a strength requirement. Having both doesn't gain you anything.
- You can use the arcane armor as a spellcasting focus for your artificer spells.
You already have one arcane armor and the tools, so gaining another focus isn't a benefit after the first.
- The armor attaches to you and can’t be removed against your will. It also expands to cover your entire body, although you can retract or deploy the helmet as a bonus action. The armor replaces any missing limbs, functioning identically to a limb it replaces.
Small to no benefit to having the outer layer unremovable also if it doesn't protect the +3, because the AC would still either be the 16 or the 18; you simply can't do the 21 by RAW anyway. So making the molten bronze skin the Arcane Armor still makes more sense. How often are your party stripped of their armor?
- You can doff or don the armor as an action.
How often do get naked in battle? Donning the armor quickly would be nice but is there a reason to ever take the molten bronze skin off anyway?