I have been doing split stays lately and it's no big deal to me. In fact, I like to try more than one resort because we stay 10-12 days in WDW. Disney considers each reservation as a separate one, so there's no carrying over a Dining Plan. If you are purchasing DDP, you are required to purchase at least a one-day base ticket for each person on a reservation. With a split stay with DDP at each resort, you will have to purchase the one-day tickets for the second resort as well. On the day you check out of POR and check-in to BC/YC you can use your first resort's dining credits (if any are left) as well as start to use your second resort's dining credits. However, you can't combine them, for example use 1 credit from each resort to pay for a 2-credit TS meal.
For tickets, we upgrade the first resort's one-day tickets to the number of days we need for the whole trip. Max on a ticket is 10 days. At the second resort we have their one-day tickets put on separate media that we take home with us. They don't expire until used, and they're legally transferrable if unused, so we put them away for a future trip or for someone who wants to buy them. As you continue your trip from the second resort, there should be no problem for their concierge desk to transfer your tickets to your second resort's KTTW card. I can't offer any info about the new RFID cards or the magic bands.