Can only married couples apply for financing together?

That's not true. They just check more thoroughly because the risk could be higher (but doesn't have to be). The terms do not automatically get worse just because an additional co-borrower is present. What could happen to the bank? The self-employed person runs their business into the ground and the employee must be able to repay the loan alone. It's no different than if the employee had taken out the loan alone from the beginning. At least that's how we proceed (we grant tens of thousands of loans to homeowners and buyers every year).

and Part 2 pays indirectly?

that would be too stupid for me

It is the case that only 1 spouse is listed in the land register and only 1 spouse is the borrower.
However, a regulation is needed in the event of separation etc. regarding
accrued gains
equity from the 2nd spouse etc.

That is not true. They just check more thoroughly because the risk could be higher (not necessarily). The conditions do not automatically get worse just because an additional co-borrower is present. What can happen to the bank? The self-employed person runs their business into the ground and the employee must be able to repay the loan alone. That is no different than if the employee had taken out the loan alone from the start.

At least that is how we proceed (we grant tens of thousands of loans each year to homebuilders and buyers).

Better conditions were only available with Part 1 as the sole borrower.

However, a regulation is needed in case of separation, etc., regarding
profit sharing
equity from the 2nd spouse, etc.