We currently need to make a decision on whether to install a center divider on our double patio door. Until recently, I was of the opinion that it was better without one. However, our GU convinced me to purchase a mullion. The main argument was that a door with a mullion is more reliable.
What is your opinion on this? And your experience?
We have a forend, if it is adjusted correctly there is no problem. The second sash is not a problem either, the lever hidden in the frame is easy to actuate. In my opinion, both doors only need to be opened properly when moving furniture, for garden parties, or for the first days of spring. Otherwise, a single leaf is usually sufficient. Long-term experience is still lacking.
At the normal 2.01 x 2.135 size, a design without a center mullion is standard; so I think your supplier's concerns are exaggerated. Only at a window size of 1.135 per sash – because of the weight of a triple glazed window – forend is not the worst solution; as long as laminated safety glass comes into play, of course. A window of this size will survive the warranty period without a faceplate, but after that.
Sliding doors also don't rattle in the wind.
I would never use a mullion on a bi-fold patio door. It always gets in the way. I would use a forend so I could get something larger (clothes horse, couch, etc.) through the door without a problem.
We have a forend at 2 meters – our GU didn't give us any warning or concern.
We have a door with a forend in our patio house where we still live as tenants. We don't use it very often, but we take the patio table out or put in new furniture, take out bulky garbage, and so on. Sometimes that would be very difficult without the forend.
The second door opens very quickly and easily with one hand (2 hinges open).
We will also have a sliding door in the new building.
We only open the other half of our double-paned patio door to bring in or take out something bulky. It really isn't hard to open. A little red lever up and the door is open. We've never regretted it.