From the official guide:
Cocos Creator is devoted to solving the problem of adapting to screens with various resolutions with one set of resources from the beginning.
As it says there, you should only have one set of resources. To support multiple resolutions, you should use Canvas and the Widget component. The canvas will make sure either it’s height, either it’s width or both (does not use the entire screen) will be respected at runtime. You should read (if you haven’t already) the documentation on Canvas and Widget.