No, the platinum and the Visa discount are not stackable. If you already have the Platinum discount, the benefits of using the Disney Visa is that you can give yourself an on board credit before you cruise - so if you think you are going to spend, say $500, on board, you call
DCL before the cruise and give yourself that as a credit. You will get 2% back as this is a Disney purchase (so less than Costco Credit Card 3% - I don't have a Costco card so I am going on what you say) but you have 6 mos. to pay off any Disney purchases over $100 that you put on the Disney card with no interest no payments (if this is important to you).
I, personally, usually buy Disney Gift Cards at a discount before the cruise (from BJ's or Sam's or even Target if you have a Target Red Card). I then use my DCL card as the card on account with DCL, but add the gift cards to my account at guest services. I can get Disney Gift Cards discounted at 5-8%, so when I spend money on the ship with my 10% Platinum discount, I really am getting in excess of 15% off in the shops and 5-8% off on drinks, etc. This doesn't include the 1-5% back I get on the credit card I used to buy the gift cards.
If I don't spend more than I anticipated -and brought in gift cards - I don't have any money go on the DCL card anyway. I will say if i do an on board booking it is nice for it to go on the DCL card because I get 2% back and 6 mos. to pay -- which is nice if I decide not to use the OBB and cancel within 6 mos. I get that money credited back and I actually never had pay it out of pocket.
Hope this makes sense.