As your scenario is quite a custom one, we would need to inspect your implementation and replicate the issue locally in order to efficiently assist you. Could you isolate the problem with some dummy data and provide us with it? In addition, please specify if you are setting the ContentURL at client or at server side. Since you mentioned "I think that maybe the RadMultiPage is getting mixed up and it's loading one iFrame's content from one page into the iFrame of another page"
, I would suggest you to try setting the contentUrl at the TabSelected at client-side or at server side.
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