You can't stack that way, but there are other ways to stack the savings. You can buy Target gift cards on Cardpool or Raise. Their discounts change on a daily basis, so you have to see which one has the better deal when you are ready to purchase. I'd say you can typically save between 6-8%. Then, once you have the Target gift card (either electronic or physical card), you go to Target.com and buy Disney gift cards. You still get your RedCard discount even though you are using gift cards to pay. I just bought enough to renew our 3 annual passes, and saved 13.5%.
There are also ways to save even more when someone runs a sale. This past week, for example:
1. Buy (2) $50 electronic eBay gift cards on Target.com for $45 each, but use RedCard and pay $85.50 total.
2. Use the $100 in eBay gift cards on eBay to buy $100 Target gift card from GiftCard Mall (cost will be $102.99 with the shipping fee)
3. When you get the Target card in the mail, go to Target.com and buy $100 Disney gift card for $95.
So your cost for the $100 card is $88.49, but you'd still have a $5 credit on your Target.com account...so you really spent $83.49. If you were to do this many times, the savings could add up! Alas, I think the sale on eBay cards is ending tonight.