While it's certainly more economical to have one winter quilt that you can vent when it's warm, it's not as easy as it sounds.
Condensation will form anywhere given the right conditions. I've had condensation form only on the top side of the tarp, only on the underside of the tarp, and on both sides
In your case, you have hot, humid air coming off you body, traveling through the quilt, and finally hitting the cooler air on the top side of the quilt. That sounds like a good formula for condensation. It's not that you did anything wrong; it just "is."
When I hike, I like to stay "comfortably cool." This keeps me from sweating and soaking my clothes. Soaked clothes eventually make me cold, even dangerously so. The same concept can be applied to sleep.