as far as i know, you cannot combine offers.
we have let the voucher go unused, but if you check the date it doesn't expire at the end of your check in (at least it didn't used to). so we used it on a future trip when we didn't get a package, just "room only" (yes it was the same year.
we used it for 3 appetizers for the 2 of us. and it was more than enough food for a lunch. also, if you ever consider a "split stay" (I'm a BIG fan of this), you get the voucher with your package for the first resort, WITH the dining plan. BUT, you USE the voucher during your stay at the 2nd resort, the room only ressie. (you bought enough park tickets with your package for the whole stay)
example: a package with the
DDP for 3 nights at coronado and 7 day tickets. then a room only ressie at POFQ for 4 nights. you still have a TS credit good till midnight, so you check into french quarter and take the boat to downtown for disney for your last TS credit. (if you're REALLY tricky, you saved a CS crredit ,cause who can eat all that food?) and when you get to FQ, you use it for a CS TO GO at the food court and put it in your fridge in your room. then you go to downtown (or boatwrights at riverside) for your last TS. the NEXT night, you nuke your TO GO CS in the microwave at the food court. the next day (or the one after that) you use the planet hollywood voucer you got from your package!! Voila! (maybe you even saved snack credits, and bought some donuts when you checked in, so you have breakfast at your 2nd resort stay!
It's really not as confusing as it sounds, and you get to experience 2 dif. resorts. Have fun!!!!
