Whoa, UpdatePanels! That takes me back. Triggering UpdatePanels to "postback" asynchronously from the client has always been a bit of a kludge. The common way was to register an AsyncPostBackTrigger with a hidden button's click event and then explicitly call its click event on the client side. However, a solution with a few less layers of indirection is to call the ASP.NET AJAX library's __doPostback() JS function.
Assuming you're using jQuery (which may be far-fetched considering you're still using UpdatePanels!), you can add an event handler to the 'focusout' event of your UserControl1 to trigger the asynchronous postback of your UpdatePanel2. I would recommend putting this JS outside of one of your UpdatePanels.
$('#userControl1').on('focusout', function() {
__doPostback('UpdatePanel2UniqueId', '');
});
I dug up a good article that explains the technique of using __doPostback in a bit more detail.
Easily refresh an UpdatePanel, using JavaScript