I would not combine the two, if you do use a action camera as dashcam you will be compromising, not least with low light performance if you get a high magapixel action camera.
Only way not to is the the 1080p action camera GIT1, but i am not sure how it will fare in the long run, if you want to be able to use it as action camera too and just snap it off the mount in the car, then you need to have the battery in the camera to be charged and ready for action, or the battery in a charger in the car ready for action.
but lipo batteries in dashcams are not recommended as the dashcams are in a place where there are high heat, and lipo batteries and high heat are not good.
Personally i would not combine the 2, get a good dashcam, then use phone or get a action camera too.