The giftcard site allows you to keep up to 5 cards stored in your account at once. Those cards can each have funds on them or not, and you can remove a card and add another one as often as you wish. You can quickly transfer the funds (and yes, it is all or nothing) from one card to any of the others. You can also transfer from a brand new card directly to any of the stored cards by entering the numbers on the back of the card and selecting the card you want the funds to go to. It's very simple.
We've purchased several special cards over the years (the giftcard site has all available card designs available and you have a wide range of initial load to choose from). I have 5 of them set up in my account, and they just stay there. I regularly purchase the generic red Mickey cards at the grocery store and immediately transfer the funds to one of the special cards.
Once we're on our trip, we each have a card and use it for everything. If someone's card gets low, it's a simple thing to transfer funds from another card on the account, and not need to carry all the cards. By the same token, should a card be misplaced, a quick transaction will transfer all the funds on the "lost" card to the one card in my account that I always leave empty just for that purpose.
Steve