Wordpress Compatibility with a Civi system currently installed on Drupal
What you're describing is difficult - but not impossible - to do.  CiviCRM has a table (civicrm_uf_match) that matches a CiviCRM contact to the corresponding CMS user id - which is pretty difficult to line up if you have multiple CMSes!  There's also settings in civicrm.settings.php that define the CMS, which is necessary for, e.g, creating a URL that's correctly formatted for the CMS in question.

You might want to see if just passing form data from Wordpress to CiviCRM meets your needs.  If you're absolutely committed to the path you're suggesting, a good place to start is here.