Hello, Paula!
You've asked a great question regarding Disney Dining Plans.
First, on the
2012 Quick-Service Dining Plan, you
cannot use two Quick-Service entitlements in exchange for one Table-Service meal.
However, you CAN use a Table-Service entitlement at a Quick-Service location
if you're on the 2012 Dining Plan. (Just look at the bottom of page 5 on the second embedded PDF link. "Some Great Tips!" states that the above is allowed.)
If you know that you're going to be eating at some of the pricier dining locations, then the Dining Plan is well worth the cost. If, for example, you are planning on eating at Cinderella's Royal Table or The California Grill, both of which use
two Table-Service entitlements, in addition to two additional character meals, then you've used four Table-Service entitlements already. (If you're staying six nights / seven days, you have only two remaining Table-Service entitlements. If staying seven
nights, you'll have three.) In order to make the most of our time in the parks, we always research park hours and special events, such as Extra Magic Hours, and make a reservation in our park of choice at a favorite restaurant during each day. This keeps everyone refreshed and recharged for more fun when we head back out into the parks. If, however, you don't want to be tied down to a Table-Service restaurant here and there, you could surely use one or two of those for a Quick-Service experience instead.
I hope that you find this information useful! Please send any additional questions you may about your upcoming trip to Walt Disney World! I'd love to help!