We haven't yet worked out a reasonable solution for adding Fastpasses across parks in advance.
When you're in the parks you can use the DLR Lines app to add Fastpasses. Once you get a Fastpass for the other park, you can go into that plan, add a Fastpass booth step for whatever attraction, and mark it as "Done" which will prompt you to input the return time. Then, when you switch parks you can Optimize via the app. I know it's convoluted and I apologize for the inconvenience. We are working on it!