merge two datatable columns horizontaly

Try this ...

I created tables similar to those you have in your post

DataTable table1 = new DataTable();
DataTable table2 = new DataTable();
table1.Columns.Add("1", typeof(string));
table1.Columns.Add("3", typeof(string));
table1.Columns.Add("5", typeof(string));

table2.Columns.Add("2", typeof(string));
table2.Columns.Add("4", typeof(string));
table2.Columns.Add("6", typeof(string));

Then I created a datatable to hold the results

DataTable dtResult = new DataTable();
dtResult.Columns.Add("1", typeof(string));
dtResult.Columns.Add("2", typeof(string));
dtResult.Columns.Add("3", typeof(string));
dtResult.Columns.Add("4", typeof(string));
dtResult.Columns.Add("5", typeof(string));
dtResult.Columns.Add("6", typeof(string));

I then take advantage of the fact that DataTable can be converted to a queryable object

var t1 = table1.AsEnumerable();
var t2 = table2.AsEnumerable();

I had to make the assumption that the "row number" was the way to join the two tables and used this to populate the result

var result = from dataRows1 in t1
             join dataRows2 in t2
             on table1.Rows.IndexOf(dataRows1) equals table2.Rows.IndexOf(dataRows2) into lj
             from r in lj.DefaultIfEmpty()
             select (new object[]
 {
    dataRows1.Field<string>("1"),
    r.Field<string>("2"),
    dataRows1.Field<string>("3"),
    r.Field<string>("4"),
    dataRows1.Field<string>("5"),
    r.Field<string>("6")
  });

foreach (var x in result)
    dtResult.Rows.Add(x);

In summary - I used a Left Join[^] on the tables and selected the columns I needed in the order you required