2 doors with separate frames = 2 separate doorways mulled together. This does not equal a 6' opening, except during the rough framing stage. You will have 2 separate completely framed openings side by side by the description given. I have seen the setup I think you're talking about (2 doors in 1 single frame to mimic the look of the patio doors). They took 2 doors and put a working latch & deadbolt on one and the receptor on the other. The receptor door also had a non-functioning handle and a slide bolt on the top & bottom for stability/security when the door was in the closed & locked position. You might be able to have a 3 point locking mechanism installed--check with your builder on this. The big thing that stands out to me when you do this is that it is hard to get the same look of the patio doors. Patio doors like you get from Pella, Jeld-wen, Andersen, Marvin, etc all have a more sleek look because the glass sits in a low profile frame. Most imitators have a bulky plastic frame that holds the glass in place. In my experience, the plastic yellows (even with a coat of paint on it) and the plugs for the screw holes never line up properly. The closest I have been able to find to a door that looks like the real patio doors from the manufacturers mentioned above is the Plastpro flush glazed door. You may want to check them out. If you want to see photos, just ask--we used the Plastpro doors on our house. Here is a link that might be useful: Plastpro Flush Glazed Doors

I spot your sister's trio of art next to your back door! They look fantastic there! (Did I suggest that? Or did I just think that was the right spot for them?) I still see Timothy's post. Maybe they vetted him and he proved he wasn't a spammer. That would be nice. We've had a rash of new posters who've come in and done nothing more than pitch a link on multiple threads. I reported on spammer who did that about a week back and the posts are now gone.

French doors usually come "pre-hung" which means they are installed in the door frame already. To do it right you need to buy doors that match the existing opening or alter the opening, including installing a new header and framing for the pre-hung doors, to fit. Altering the doors to fit the existing opening is not a good plan.

I would call in another Marvin Dealer as if you were starting from scratch. Have them come in for a consult and a second estimate. Ask the question and see what he/she says. Even if you’re happy with the first dealer, dealers can charge differently so at least you’ll have something to compare apples to apples with. I actually have two appointments scheduled for Sunrise windows next week, but I’m not actually sure which dealer I’ll end up with. I have to say though both my mother and my sister had the french doors installed. They weren’t Marvins but they were extremely happy with that style.