Do you mean a 42" cabinet box with 2 21" doors? Or a 42" cabinet box with a single door? The single door would be extremely wide and impractical, so I'm assuming you mean a single box with 2 doors. I have a few wall cabinets like that, but not that wide.

If you must, do a 42" cabinet with two 21" single doors. I actually prefer a 36" cabinet with two 18" single doors. A very wide door, when open, is a hazard.

I prefer a cabinet that had no center supporting post.

We have a (1950s) wall cabinet that is about 6' long, all one box, with 5 separate 14" doors. The small doors are nice in terms of not hitting your head on them, and there is more storage than if we had a larger number of smaller cabinets, but the big down side is that the shelves are not adjustable since they have to run the length of the box.

A 42" wall cabinet will still have adjustable shelves. Sounds like Issac's cabinets were custom possibly. They probably did fixed shelves to strengthen the cabinet. A 42" cabinet needs more support (ie. screwed to ceiling, side cabinets, back wall..e.tc). Storage wise, the 42" cabinet has more. The 42" will cost less.

Are your cabinets framed or frameless? usually frameless only goes up to 36" wide in mid grade lines. Are they partial, full or inset doors? See if you an do one cabinet with a partition down the middle for strength. And adjustable shelves. It will have good stability and two separate spaces to store things. Or if you have good stability...go with the full shelves across. If you do 2-21 cabinets you will have a joint line where they are screwed together. If you have partial or inset...go with ONE cabinet.

42" (~41") shelves sag unless they are beefed up.

thanks so much for the input

Even in framed cabinets, 2 21” cabinets will hold much more weight than a single 42” cabinet. A 42” will hold more volume. It depends on what you want to store there. Lightweight plastic storange? A 42” framed is fine. Your cast iron Staub collection? You’d want 2 21” with an upgrade to the 3/4” shelving, possibly also upgrading with an added shelf nosing treatment in the field.