In a converted hosue where the door is shared to both the downstairs and upstairs flat, is is possible to buy locks where both flats can just use one key each?
Key one opens main door and flat 1
Key two opens main door and flat 2?

If so where can I get it?

yup. exactly what I have.

1 key for each flat can open main door, door to their section/floor, and that flat. But can’t open door to other sections/floors, nor other flats/storerooms (obviously).

and each key can obviously open other communal locks (eg door to garbage shed etc.)

and the caretaker can probably open all doors to all sections of the building with one key (but not flats). and no doubt there’s a master key that will open all doors.

get them from a locksmith 😉 mine is all standard eurolock format lock I think.

Is it that hard having 2 keys instead?

My Granny used to have that set up. The keys were expensive and had to be obtained from Germany apparently. The combination of old folk and expensive replacement house keys is a bad idea.

Is it that hard having 2 keys instead?

Quite distressing and debilitating for the tenants yes.