Filter record based on multiple TextBoxes in ASP.Net

There is a search panel. The panel contains 2 input types ie. Keyword and category. There is one button.

When i enter only a keyword suppose say keyword AUTO and clicked on search,it redirects to the searchmain.aspx page and it searches the keyword AUTO of all categories. When i enter only cateory suppose say Automotive and clicked on search,it redirects to the searchcat.aspx page and it seraches the category.

i want in such a way that when user enter both keyword and category,it searches for the word in a selected category.

Link: http://alpha.dubaiexporters.com/Default.aspx

     protected void doit(object sender, EventArgs e)
     {
        if (!string.IsNullOrEmpty(txtkeyword.Value))
        {
            try
            {

                String mainsearch1 = null;
                String cat = null;
               
                //  mainsearch1 = txtserach.Text.Trim();
                mainsearch1 = txtkeyword.Value.Trim();
                if (mainsearch1.Length > 2) 
                {
                    Response.Redirect("searchmain.aspx?mainsearch=" + mainsearch1 + "&querylevel=1");
                }


            }
            catch (Exception ex)
            {
            }
        }

        else if (!string.IsNullOrEmpty(txtserach.Value))
        {
            try
            {

                String cat = null;
                //  mainsearch1 = txtserach.Text.Trim();
                cat = txtserach.Value.Trim();
                if (cat.Length > 2)
                {
                    //Response.Redirect("searchcat1.aspx");
                    Response.Redirect("searchcat1.aspx?cat=" + cat);
                }
            }
            catch (Exception ex)
            {
            }
        }
    }
}