If there's an exchange rate between them, they're not separate economies.

Personally I don't think any contortion is needed to explain why gold and high-end items are separate economies. Anyone who needs or can make a +5 sword is sufficiently powerful that they can buy, seize, or just magically produce any conventional goods they need. They're not going to give away their +5 swords for anything they can get easily, so only other magic items or goods of similar rarity can be used to pay for items on this level. "Magical ore that replaces material components" is a pretty good currency choice for this, but by making it a currency that can buy high-end magical items it's pretty much by default not going to be available for any amount of gold.