The problem is your asking about part arming and no one knowing the details of the system you have, or what you want to actually achieve.
Part arming options vary from system to system and how you want it to work, may determine the best option.
two keypads or panels with built in keypad and a second keypad still apply but are they needed.
realistically I am guessing you don't want to go down stairs at night in your pyjamas and so you want to set it from within the flat, but when your going out of the flat you want to arm it when you leave the front door.
With an App uit can all be done from the smart device (compliance would be an issue but as its bells only it doesn't really matter), but if your relying on keypads, then how you want to set and unset determines on the system and how its programmed and used.
You could treat the downstairs door and hallway as a separate entity to the flat, but still arm in part set from the flat, still need two keypads for that.
with one keypad in your flat you could arm part set from that keypad and full set from that keypad, and hae the doorstairs door as the final exit so in full set it has to see the door open and close.
The issue then is starting the entry and getting to your flat to disarm if thats the only place there is a keypad.
In an ideal world how do you want it to work, where do you want to arm and disarm from when system full set and part set.