How to return two list values from web service to asp.net project

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ASP.NET

C#3.5

webservice

Actually from my web service i am return the drop down values and binding in asp.net but in my asp.net page i am having two drop downs with different values now what i am doing is i am calling two different methods to return two lists but i am thinking to call only one method and return two different list from one call to service is it possible now Below is my code.

public List<GrpUsersResp> GroupAllReferrals(GrpUsersReq Req)
        {

            using (SqlConnection con = new SqlConnection(con1))
            {
                SqlCommand cmd = new SqlCommand("Sp_Groups", con);
                cmd.CommandType = CommandType.StoredProcedure;
                //cmd.Parameters.Add("@PNMode", SqlDbType.NVarChar, 50).Value = "Groups";
                cmd.Parameters.Add("@GMode", SqlDbType.NVarChar, 50).Value = "GrpAllRefUsers";
                cmd.Parameters.Add("@GroupID", SqlDbType.NVarChar, 50).Value = Req.GroupID;
                SqlDataAdapter da = new SqlDataAdapter(cmd);
                DataSet ds = new DataSet();
                da.Fill(ds);
                
		DataTable dt = ds.Tables[0];
                foreach (DataRow dr in dt.Rows)
                {
                    GrpUsersResp cls = new GrpUsersResp();
                    cls.UserID = dr["userid"].ToString();
                    cls.UserName = dr["UserName"].ToString();
                    GrpAllRefUsers.Add(cls);

                }

		DataTable dt = ds.Tables[1];
                foreach (DataRow dr in dt.Rows)
                {
                    GrpUsersResp cls = new GrpUsersResp();
                    cls.UserID = dr["userid"].ToString();
                    cls.UserName = dr["UserName"].ToString();
                    GrpReferrals.Add(cls);

                }

            }

	//But Here i want return two values how is it possible
            return GrpAllRefUsers;
        }