Double doors with or without a central mullion - difference?

For our new building, double doors without a mullion are planned from both the kitchen and the living room to the outside. What should I imagine by this? There must be something in the middle after all. The glass does not just hit glass. Due to the size of the windows, the manufacturer does not provide any warranty if we choose triple glazing because they would then be too heavy. So we are only using double glazing.

Patio door with mullion: the window trim, that is the window frame, is of course there, but in the middle of the window opening there is no mullion. Advantage: 2 meters continuously open. You should not go wider than 2 meters. ?

Double doors without mullion

People rather say "double-leaf with rebate", and here the rebate of the leaf that opens second is simply arranged in the opposite direction. That means the leaf that opens first closes rebate to rebate against the other in the middle of the entire opening. By mullion one means a fixed post between the two opening leaves, which, in my opinion, largely renders a double-leaf door absurd.

Due to the size of the windows, the manufacturer does not provide a warranty if we choose triple glazing because then they are too heavy.

Wow, how huge are they supposed to be?

Advantage: 2 meters continuously open. You should not go wider than 2 meters. @11ant?

I recommend (if you like it classically symmetrically divided) the width of 226. The rough opening width of only 1 meter is a bottleneck with a tray or laundry basket, at least insufficiently comfortable. If you only have the width of 2 meters, I recommend accordingly an asymmetrical division, so that the first leaf to be opened can have a width of 113. I only recommend sliding doors from a width of 251 upwards. I am also happy to give more concrete recommendations if the overall situation (whole room with all windows) were shown here.

I also don't understand why triple glazing is not possible.

It is important to know, as already mentioned in passing in the other posts, that you cannot open both sashes independently of each other. Therefore, there is always a "leading" sash. Whether this is left or right should be carefully considered.

Unfortunately, we found out that our window company also only equips these doors with one handle (on the "leading" sash), the other is then unlocked inside the frame and can be opened. We are in the process of getting mentally used to the asymmetrical view.

equips these doors only with a handle

That's normal, isn't it?

I only know it that way too. We have a Stulp terrace door and a window like that in the kitchen. We were asleep in the kitchen, but you get used to it.