Looping through two identical DataReaders produces an error

    protected void BuildCombinations(DateTime CurrentInterval)
    {
        /*
        Function BuildCombinations()
            Delete all records in the COMBINATIONS table
            For each Member (J) of the active Group where Unavailable = 0 && Assigned = 0
                For each Member (K) of the active Group where Unavailable = 0 && Assigned = 0
                    If (J) != (K)
                        In COMBINATIONS table:
                            Set ParticipantA = (J)
                            Set ParticipantB = (K)
                            Set LastTeamed = IntervalStartDate - DateTime LastTeamed (from SCHEDULE table)
        */
        
        int activeGroup = Convert.ToInt32(Session["GroupID"]);
        DeleteAllScheduleCombinations(activeGroup);
   
        string ConnectionString1 = ConfigurationManager.ConnectionStrings["dbCS"].ConnectionString;
        using (SqlConnection con1 = new SqlConnection(ConnectionString1))
        {
            SqlDataReader reader1;
            SqlCommand cmd1 = new SqlCommand("spClientGroupMembersUnteamedGet", con1);
            cmd1.CommandType = System.Data.CommandType.StoredProcedure;
            cmd1.Parameters.AddWithValue("@GroupID", activeGroup);
            con1.Open();
            reader1 = cmd1.ExecuteReader();

            if (reader1.HasRows)
            {
                while (reader1.Read())
                {
                    int m1 = reader1.GetInt32(0); //ParticipantA

                    string ConnectionString2 = ConfigurationManager.ConnectionStrings["dbCS"].ConnectionString;
                    using (SqlConnection con2 = new SqlConnection(ConnectionString2))
                    {
                        SqlDataReader reader2;
                        SqlCommand cmd2 = new SqlCommand("spClientGroupMembersUnteamedGet2", con2);
                        cmd2.CommandType = System.Data.CommandType.StoredProcedure;
                        cmd2.Parameters.AddWithValue("@GroupID", activeGroup);
                        con2.Open();
                        reader2 = cmd2.ExecuteReader();

                        if (reader2.HasRows)
                        {
                            while (reader2.HasRows)
                            {
                                int m2 = reader2.GetInt32(0); //ParticipantB
                                if (m1 != m2)
                                {
                                    int lastTeamed = GetDateLastTeamed(CurrentInterval, m1, m2);
                                    WriteScheduleCombination(activeGroup, m1, m2, lastTeamed);
                                }
                            }
                        }
                        con2.Close();
                    }
                }
            }
            con1.Close();
        }
    }

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USE [Scheduler_Dev]
GO
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
ALTER PROCEDURE [SchedulerSA].[spClientGroupMembersUnteamedGet] 
	@GroupID int
AS
BEGIN
	SET NOCOUNT ON;
	SELECT Id 
	FROM ClientGroupMembers
	WHERE GroupID = @GroupID AND Unavailable = 0 AND Assigned = 0
END

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USE [Scheduler_Dev]
GO
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
ALTER PROCEDURE [SchedulerSA].[spClientGroupMembersUnteamedGet2] 
	@GroupID int
AS
BEGIN
	SET NOCOUNT ON;
	SELECT Id 
	FROM ClientGroupMembers
	WHERE GroupID = @GroupID AND Unavailable = 0 AND Assigned = 0
END

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