IIRC neither game can be in free trial status for the discount to work.
Ah here we go:
Link a PlayOnline ID with an active FINAL FANTASY XI Content ID to a Square Enix account with an active FINAL FANTASY XIV service account to discount your FINAL FANTASY XI monthly service fees from ￡8.99 to ￡5.00 or from €12.95 to €7.00 or $12.95 to $7.00 .
* These perks will remain available until January 2011.
* This FINAL FANTASY XI monthly service fee discount applies to users paying by credit card.
* No discount will be applied to a month’s service fees if FINAL FANTASY XIV is in a free trial period on the 26th of the month.
* Additional fees will be incurred for additional FINAL FANTASY XI Content IDs. Relevant link. Edited, Oct 23rd 2010 12:12pm by Redyoshi