Code :
<form id="form1" runat="server">
<div>
<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
<asp:TextBox ID="TextBox2" runat="server"></asp:TextBox>
</div>
</form>
if I click on an input field and I press Enter, it submits nothing. Why? And how can I decide which page to call, pressing Enter, for each input box?
You can put both boxes to separate Panel and set DefaultButton for each Panel.
UPDATE DefaultButton will work if you use Button, LinkButton works only in IE, there are some workarounds :
set linkbutton as default button for asp:panel in asp.net
Reusable Page_PreRender function in asp.net