Also curious about this. You cannot physically check into your 2nd hotel until they system has checked you out of your 1st hotel. We had an issue with this a few weeks ago. I went to the front desk at YC to physically check out of the resort. I was told everything was fine. I was checked out.
Then went to BI to check in. There, I was told that I could not check into BI until I had checked out of YC. I told them that I had indeed checked out of YC. They said the system did not have me checked out so I would have to wait.
It seems that ideally we should be able to use our DP credits on the morning of check in, but the system does not always work ideally. I can see the split stay foiling plans.
I hope not. We also have a split stay in July and I am seriously considering booking
DDP for the second part (with one adult and two children, it seems quite a good deal- a way to do Akershus and Fantasmic package and other dining experiences I would not consider without the plan).