XML : Auto increment a number field in text?

I have 2 resources file with the following format (file.xml):

  <message>      ...      <id>1</id>  <message>  <message>      ...      <id>2</id>  <message>    

And when I merge 2 file, I want the second file have the "id" field increment begin with the last number of "id" of the first file. Because the number of line in 2 file is very big so I can't do it by hand.

Is there any simple way to do that?