Are two non-exchangable currencies possible in an MMO?

Imagine an Adventure Coin and a Merchant Coin.

Adventure coins can be spent on training and equipment, at in-game shops, but cannot be transferred between players.

Merchant coins can be transferred between players, and are used for investing in land, ferries, mills, etc.

Do any existing MMORPGs have such a system?

How can this work in the face of "bad money drives out good"?