How to append two datatable columns into one

The problem is that Merge doesn't do what you want: it appends one table to another with a similar schema.
So Merge will give you this:

1   a
2   b
11  Kochi
21  Bangalore

It can't "combine columns" to form a new table, because it has no idea how the rows are related - and in fact there is nothing in your tables which in any way relates two rows together.

To do what you want, you will have to create a new third table with four columns, and manually add the data to each row. (And hope like heck they have the same number of rows).

At a guess, your data organisation is flawed, and needs some serious looking at instead of the "combine" operation you are trying for.