I'm using RadAjaxManagerProxy in UserControls (.ascx) and a RadAjaxManager in Page (.aspx). It's working, but when I try refresh two user controls at same time the first POST request is canceled.
That is to be expected unless you set the RequestQueueSize to a certain limit. So in your case, you can set the RequestQueueSize of your RadAjaxManager to 2, in order for the second request to not cancel the first.