DECLARE @x XML = '
<Root>
	<Student>Jhon </Student>
	<Student> Luka </Student>
	<Post>1</Post>
	<Post>2</Post>
</Root>'

SELECT @x.query ('
	for $i in (Root)
	let $s := $i/Student
	let $p := $i/Post
	return
		<Root>
			<Students> {$s} </Students>
			<Posts>{$p}</Posts>
		</Root>
')

/*
Produces: 
<Root>
  <Students>
    <Student>Jhon </Student>
    <Student> Luka </Student>
  </Students>
  <Posts>
    <Post>1</Post>
    <Post>2</Post>
  </Posts>
</Root>
*/