Nothing is ever simple! :\
By 6 web servers does that mean 6 Moodles?
If so, each moodle needs it's own cron job as each moodle instance has it's own datadir and own code ... cept maybe in how you've described your setup ... are the 6 web servers (moodles) running off the same 'shared code base'?
And one more suggestion to add to the confusion ...
Rather than outputting cron to null - which means maybe no one sees it or the cron job results get sent to an EMail that no one checks , consider redirecting to a log -> /var/log/moodle1-cron.log and /var/log/moodle2-cron.log for the 2nd cron job (one per moodle server).
The individual cron jobs would have to execute in code/admin/cli/ as that php script is tied to task for that instance and reads config.php file for variables related to that moodle instance.
Then one could watch real time output of those cron jobs via:
tail -f /var/log/moodle1-cron.log or install multitail and setup one 'watchcron' multitail script that points to all the /var/log/moodle#-cron.log's being kept on server.
'spirit of sharing', Ken