IIRC, FP+ treated completely on-property split stays as a single combined reservation, and treated on/off-property combos as separate on-property and off-property bookings. However, the way I understand it, the dining reservation system currently treats any split stays as separate reservations.
It's possible the new system just reverts to the way FP+ did it, but it might also work the way current dining reservations do. Since they've implemented split stays both ways, it's honestly anybody's guess as to what the new LL system will do.