I Added the below code in Page_Load:
if(!IsPostBack)
{
TextBox1.Attributes.Add("onkeypress", "javascript:if (event.keyCode == 13) { __doPostBack('" + btnAdd1.UniqueID + "',''); return false; } ");
TextBox2.Attributes.Add("onkeypress", "javascript:if (event.keyCode == 13) { __doPostBack('" + btnAdd2.UniqueID + "',''); return false; } ")
}
works like charm!!