They used to, but to my knowledge there is nothing like that now.
XI is their most profitable title to date, dropping or reducing the sub would be silly.
except it is what they did from day 1
http://www.siliconera.com/2010/08/17/final-fantasy-xi-subscribers-start-final-fantasy-xiv-with-golden-shoes/
Quote:
Finally, if you decide to keep your Final Fantasy XI account active, Square Enix will give you a $5.95 discount making the monthly fee for Final Fantasy XI $7.00.
Give me a CoP era server (the last time I seriously played the game) and I'll sub. That means no level sync'ing, no page burning parties, cap @75, no /dnc making solo'ing a joke. I want to grind Valkurm Dunes til my eyes bleed! /hardcore
They did that day 1 in XIV 1.0 and subsequently ended that just as quickly as they started it - This isn't true as unless you're a 1.0 player you cannot get Hermes Sandals anymore.
I still pay $14.95/month for XI and 9.99 for XIV, so they most definitely do not have the dual subscription option active.