(4) "Kim would not give into her demands".
(5) "He gave into the pressure".
(6) "Ed handed his paper into the teacher".
The composite form has a number of different uses, as can be seen in (4) and (5), where "give in to" is a verbal idiom meaning 'succumb', and (6) where "in to" indicates movement and transfer of something to a recipient.
Now consider your two examples:
(7) "It's a house that's been divided in to into two separate flats".
(8) "There is one flight of stairs to take in to into consideration".
In (7) a change of state is indicated, just as in (2), so the compound "into" is correct. And (8), an idiom, means to take into account, and again the compound form "into" is correct.
(1) traditional grammar treats the "in" seen in examples (4) - (6) as an adverb, but modern grammar analyses it as a preposition.
(2) the "to" that is used to introduce infinitival verb phrases, as in "Ed just popped in to check on something", is a subordinator, not a preposition.