Not quite sure I understand the question as worded, or the replies, but here's my take.
I don't know about midnight, but I have sometimes arrived as early as 6:30am to my 2nd hotel when doing a split stay.
At that hour (or any times earlier), bell services is open, and you can drop off your bags with them. Before 6:30am though, nothing is really open at the hotel. If you have done online check-in, then you don't need to visit the front desk. If no online check-in, the front desk is open. You are allowed to park that early. I think the pool is not open that early, but you can swim as soon as it is open. You can 100% do early entry in MK that morning, and drop off your stuff before going to MK.
If your room was vacant the night before, it might be ready fairly early. I once had my room ready at 7am, but usually it will not be ready until a later time. There is a good chance it will not be ready until 4pm.
If your 1st hotel was a WDW hotel, you need to be out of that room by 11am, but you can also go back to that hotel for the rest of the day to swim, park, etc.