Disney does NOT allow the use of a 2-Fer aka Pay for Disneyland, get DCA for FREE! on the same day in 2006. If you try and do it, the CM at the turnstile will send you back to the Ticket Windows to upgrade to a 2 day ParkHopper for $20 more.
The turnstile computers WILL catch it, and you just can't do it anymore... Sorry!