From what I'm getting, dPoW involves taking already-mined blocks from one blockchain (in this case, XMR's) and then notarizing it into another blockchain with better security (such as Bitcoin or Ethereum) for extra security measures; while merged mining is about mining multiple blockchains at the same time, such as mining BTC and XMR together.

It's like the difference between doing two jobs at once compared to getting your work double-checked by your boss.