Terrace doors/windows with stulp - with or without flat threshold

Hello everyone,
our window installer is giving us the choice of whether we want the patio doors or floor-to-ceiling windows (with [Stulp]) with or without a flat threshold.
These are only 2 cm high and cost 80 euros more per window element. What bothers me, however, is his remark: "It is worth mentioning that with a [Stulp] design, in the area where the two sashes meet, it can happen during driving rain and wind on the window that some water is pressed into the room. This design is basically two patio doors side by side, whereas previously only one of the two sashes was planned."
I don’t understand the last sentence, but water in the room would be bad, right?

What are your comments on this?

Is the neighbor's balcony by any chance above your terrace?

Either way, I would do it like this with a mullion and flat threshold.

We have "flat thresholds," so far a very good decision. Until now, no water has entered the interior. As cautious people, we have installed drainage channels outside (the civil engineer found it unnecessary, but the authors of the relevant standards insist on it).

Den letzten Satz verstehe ich nicht,

Same here!

The terrace is only slightly covered by the neighbor's balcony and otherwise open. Whether a drainage channel is planned, I need to inquire.

We also have a flat threshold and have already had water in the living room.

In 2 years, maybe 100ml once. No problem with tiles, but carpet, laminate, or parquet do not really like that.

We also have a flat sill and have already had water in the living room.

Do you have a drainage channel outside?