I found the easiest way to get back to back FP+ times, for me, was to originally book all of the FP+ at the *opposite* time of day that I wanted them, and then modify them to the times I was looking for. (For all but 7DMT and A&E - for those two, I'd take the best option).
For example, if I were spending the day in MK and I wanted late morning/midd-day FP+, I would look at the 3 pre-made options
MDE pops up (usually A B and C are morning/afternoon/evening times), and I'd choose the set of evening FP+. Then I'd immediately go in and modify the times to the times I actually wanted each FP+ for.
This way, MDE wasn't blocking me from seeing availability simply because one of the FP+ I had was during that time. That's one of the flaws in the system, IMO. It should show you ALL availability, and if you choose a time that overlaps another FP+ it should prompt you to keep one and modify the other.
If I took the options MDE first gave me in the time slots I originally wanted, I found it much harder to modify them to back-to-back slots because it wouldn't show me availability (even if there was availability) in slots where other FP+ were booked.