You can use LINQ, Its a C# code you can find the corresponding Vb code
here
var query =
from table1 in Datatable1.AsEnumerable()
join table2 in Datatable2.AsEnumerable()
on Datatable1.Field <int>("Studentid") equals
Datatable2.Field <int>("Studentid")
select new
{
Studentid=
Datatable1.Field <int>("Studentid"),
Marks =
Datatable1.Field <int>("marks") + Datatable2.Field <int> ("marks")
};