If I understand correctly, you want to process the data in the form both in web2py and in spip.
1. You could do normal processing in spip, but add an event on form submit that sends an ajax query to a special function in the web2py server.
2. Maybe, just maybe, the spip code will not treat post and get variables differently, and if doesn't have any form security mecanism like web2py does, you can just navigate to a url
your_domain.com/spip.php?article18&name=john&surname=doe3. Otherwise you can use the wonderful python request library to interact with just about any website, spip or not.