I'll say what has been my experience. I use Garmin CN and MMI v7 maps on an ipod touch, Asus M10 and GMPC. As MMI maps have the higher priority value; when both are selected, then both display and routing will be from the MMI map. If you need the CN display and routing, you'll need to deselect the MMI map. The Asus M10 has a peculiar problem with the map source selection screen; as the display of the MMI v7 map name is very long, the program crashes when the selection screen is loaded. This is not a problem in GMPC, Garmin on IOS or GMXT on other WM phones.

To sum it up, I've not faced any problems with running the navigation with both maps selected (caveat: I've no experience of how nuvis will behave or of other overlapping maps). One thing you should not do ever is change map selection after route has been calculated, as the road co-ordinates do not exactly coincide for the two maps and you'll see the route marked outside the road sometimes!

However if you merely need the POIs from the other map, there is no need to select both maps. POIs from the unselected map also show up on the selected map and route correctly.