Good point MS, thanks for mentioning that. In the WMS Explorer Add-In, you can select multiple layers at one time by using the shift key or the control key (command key on mac) when selecting. This will generate the request with a comma-separated layer list. If you are adding the layer name manually in the background map dialog just be sure that you don't have any spaces in your list. So something like this: layer1,layer2,layer3, with no leading or trailing spaces either.