try like this
DataSet ds=new DataSet();
ds.Tables.Add(Table1);
ds.Tables.Add(Table2);
ds.Relations.Add("TheRelation", Table1.Columns["a"], Table2.Columns["a"]);
Table2.Columns.Add("b", GetType(System.String), "Parent.b")
and see below link you may get an idea
http://stackoverflow.com/questions/1262383/merging-2-data-tables-in-vb-net[
^]