function keyDown(e) {
    if (e.keyCode == 13) {
        var targ = e.srcElement;

        if (targ.id == "txtIsoSearch") {
            var button = document.getElementById('<%= btnIsoSearch.ClientID %>');
            button.click();
        }
        else if (targ.id == "txtGoogleSearch") {
            var button = document.getElementById('<%= btnGoogleSearch.ClientID %>');
            button.click();
        }
        return false;
    }
}

//asp-side
<asp:TableCell><asp:TextBox ID="txtIsoSearch" runat="server" onkeydown="return keyDown(event)" /></asp:TableCell>
<asp:TableCell><asp:Button ID="btnIsoSearch" runat="server" Text="Search" OnClick="btnIsoSearch_OnClick" /></asp:TableCell>