how to combine the columns of datatable to dataset

See more:

ASP.NET

i want two merge the two columns of datatable to dataset and want bind to gridview.
i try in following two ways but it merge datable to dataset with different table.

DataTable DtAudit = new DataTable("AUDIT");
DataSet ds = null;
ds.Tables[0].PrimaryKey = new DataColumn[] { ds.Tables[0].Columns["AR_Code"] };
            DtAudit.PrimaryKey = new DataColumn[] { DtAudit.Columns["AR_Code"] };
            DataSet dsNew = new DataSet();
            dsNew.Merge(ds);
            ds.Merge(DtAudit);

And the second way

DataTable DtAudit = new DataTable("AUDIT");
DataSet ds = null;
foreach(Datacloumn dc in DtAudit)
{
ds.Tables[0].columns.Add(dc)
}