Update Panel + text box

I have been using ASP.NET for some time but I am new to AJAX

So I am trying to updtae the content of TextBox2 when the user changes TextBox1 and when it loses focus, nothing fancy, 

To avoid a whole page postback I am using Update Panel

When both textboxes are in the same update panel I am able to update the content of the second box (since both are refreshed by the autopost back event)

Interestingly, when the TextBox2 is not in the same UpdatePanel this is not happening because TextBox2 is not refereshed

So if TextBox2 is not in the same update panel, how do you update the content?

I guess one solution woud be wraping TextBox2 in another updatepanel and invoking some event to update that second panel? How do you do that or is there another solution?

Thx