Basically - yes, you could have multiple RadWindowManagers per page. In such case however, you need to take into consideration the following:
1. All RadWindowManager's functions (radopen, radalert, radconfirm, radprompt, GetRadWindowManager, etc) are always using the first rendered RadWindowManager on the page.
2. Every RadWindowManager "knows" only the RadWindows that are declared in its Windows collection.
This means that if you have a RadWindow2 declared in RadWindowManager2, and you use something like radopen(myUrl, "RadWindow2");, radopen will use RadWindowManager1 and will open a new RadWindow with the settings taken from RadWindowManager1. To avoid that problem, when you have multiple managers on a page, you need to get a reference to the correct RadWindowManager first and then call its methods.
var manager = $find("<%= RadWindowManager2.ClientID %>");
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