Thanks for the update!
I'm planning a potential split stay myself and I'm almost tempted to make a "dummy" onsite room-only reservation covering the same dates as our split stay (once it's booked), solely for the purpose of being able to make ADRs and FP+ for the whole trip at one time (and then canceling the dummy room reservation, penalty-free, once those are made) -- the better to make sure that I can make a decent dinner reservation for my daughter's birthday, which happens to fall during the second half of the split. It's ridiculous for WDW to, in effect, penalize onsite guests who stay in more than one resort, by denying them access to ADRs and FP+ during the very booking windows that it promises in all its advertising for those staying onsite (none of which ever warns guests, "...but only if you don't switch to another onsite hotel"). But I realize it's really an IT-not-keeping-up issue, as opposed to a Disney-doesn't-like-split-stays issue...