Two-family house - Two parties, two financing loans?

It is probably possible to issue a termination for personal use for one's own children as well, so that should work.

Of course, now the full notary costs of X euros are incurred and then again the notary costs of Y euros upon sale (which amount to half, since it's half the purchase price). In the end, you have paid a few thousand euros that you could have saved. But if that is the only solution, there is no alternative.
You could directly carry out the division at the time of purchase later, although I do not believe that this saves notary costs, since surely the purchase contract + division will be billed separately.

Next week there is another appointment at the currently lowest-interest-rate bank. Maybe they still have a suggestion or will finance it with two loan agreements.

Notary fees for the apartment amount to double the cost completely. However, they would have been incurred at least partially with the other option as well.

What I just thought of:
Parents + partner buy the house at 50% each, parents take out the two loans themselves. Then splitting, the partner takes out a loan for the apartment and her parents pay off the loan with the variable interest rate. That way, no sale would have to take place anymore and you save the double notary fees.

So, the offer from the local bank is here. It is slightly cheaper than the cheapest bank at Interhyp. Initially calculated with 100% financing. KfW would be 0.82% for 10 years + the bank with 1.1% for 15 years or 0.8% for 10 years. Two loan contracts are no problem, with the division the mortgage will then be "divided" between the two apartments.