Great Eirikur, 

didn't think about that  😉
Now it works as I want.
Thank you

For those interested :
declare @head1 table
(head_id uniqueidentifier)

Maybe interesting to note that the 'for xml' statements are not allowed/supported inside union all queries

Alas, the resutl I get then is thi
<master>
<alldata head_id="6DBD64CD-44CE-425C-A105-BE1900299550">
  <sub>
      <own_id>10</own_id>
      <own_field>Sub1 field</own_field>
      <own_id>11</own_id>
   ...

Sure,
This is what the quey returns now : 

<master>
<alldata head_id="C6DF494E-4DA7-4726-8288-BB463CB96834">
  <sub>
  _{<own_id>10</own_id>
   <own_field>Sub1 field</own_field>}
  <sub>
  ...