This can be achieved by a FLWOR operation.

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>

*/

This is one of those scenarios where the FLWOR operation is quite handy. I just added this example to the XQuery Labs