Technically you have to have some form of 'stretch' enabled. That way you can move your mouse from one monitor to the other. I can set up my duals just under Display Properties, Settings tab. From there you can setup your primary display and position where you want your secondary. Can you set their resolutions from there?
I use a horizontal stretch, but you should be able to set each one to their respective resolutions..
And yes it maximizes to just one monitor at a time.