If you're good with PHP and programming concepts you probably could. As in it's not out of the realm of possibility.
The tricky parts as I see it would be:
1) Figuring out what would trigger your PHP code to run
2) Communicating between two servers on possibly two different domains (cross domain communication)
3) Making sure your code is secure (you don't need someone to trick your code into loading up a malicious XML file)
4) Making sure your code has proper error checks and validation to make sure everything runs correctly (so it doesn't crash your server if something fails or goes wrong)
And of course, the search engine penalty for having the same content posted on two different websites (i.e. if Google and other search engines catch you trying to bump up your rankings this way - your sites may be delisted from Google).