The both options you described are effective enough in case the correct Ajax settings are added on the page.
The best option is to always keep the RadAjaxManager on the MasterPage no matter how the ajax settings will be added. You could use the FindControl() method to locate all the controls which are using the mentioned master page in the application tree and ajaxify them on the MasterPage.
Or another approach could be instead of adding RadAjaxManagerProxy in all other pages just use the RadAjaxManager.GetCurrent() method to get instance of the main manager and continue adding programmatic settings in all independent content pages.
See the online resources below for more information on this matter:
I hope this helps. Do not hesitate to contact us back in case further questions arise.
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