Yes, both of the mountains are above the same subduction zone. The Juan de Fuca plate is sliding under the North American one, and the Cascades are basically above the region where that plate becomes molten below us. That's why that last big quake here was so deep - thankfully. The Olympics are their own plate, but I'm not certain about the mechanics of that.
May I ask you a question because frankly, I don't know squat about volcanoes?
For the exeption of ones in the sea, are land based volcanoes typically located in a moutain? Can they be underneath the flatlands?
And, is it possible to have more volanoes than those that we are aware of?