Compare Datatables - Matching Rows and Columns by Id

Assuming there are no nulls in either datatable and that there will always be a match in table2 for each D_ID value and column name in table1, try this (untested):

DataTable table3 = table1.Copy();

foreach(DataRow row in table3.Rows)
{
    string D_ID = row["D_ID"].ToString();
    foreach(DataRow row2 in table2.Rows)
    {
        if (row2["D_ID"].ToString() == D_ID)
        {
           foreach(DataColumn col in table3.Columns)
           {
              if (col.ColumnName == "D_ID" || col.ColumnName == "Split") continue;
              double val1 = double.Parse(row[col.ColumnName].ToString()); 
              double val2 = double.Parse(row2[col.ColumnName].ToString()); 
              row[col.ColumnName] = (val1 - val2).ToString();
           } 
        }
    }
}