Overlapping is not a problem if the map data in the overlaps is identical. Although any data changes are likely to be minor [or even non-existant], problems may occur if there are data differences using 2 versions which have the same precedence and with both selected as you described. Best to disable the chart not being used at the time to be sure. If navigating through the overlap from one chart's exclusive coverage to the other's as i think you're wanting the plotter will probably simply use the later data in which case there won't be any conflict anyway. That particular 'smart' does vary with Garmin devices though. For example some automotive devices have been shown to freeze or crash if faced with conflicting data in 2 versions of the same CN map in certain circumstances.