To avoid having to set up multiple cron lines - here's a script that will read your drupal install(s) and call cron on each one found.
Not sure where I saw the code (rss feed) - but the comment was present in the downloaded code - so thats a pointer to where it starts.
Changes I've made - the dir match is changed from *_ to _ so that it will follow symlinks. It also handles multiple drupal installs. Timeout wrapping added - one of my sites takes a long while to return.
Feel free to comment with fixes or suggestions to make it better.