You didn't exactly specify, but it sure sounds like an existing garage you'd like to modify. There is much more involved to widening an existing garage door than you might think. A new header, cutting the foundation walls and portion of slab, pouring new slab, are the walls combatting shear for the structure, the width of the driveway, etc.

A 20' door is offered for a residential stamped panel, but 18' is the more common oversized width chosen. What is critical is the amount of space adjacent to the cars, not so much the space in between (you may walk by them both to get outside, access shelves, whatever many times a day vs getting out of 1 of the cars 2 or 4 times a day). Consider that a typical space in a parking lot is 9' wide.

New home construction. We are in the design phase.

The wider the "pillar" in between a set of single doors, the odder the proportion IMO. Since you are starting from scratch, put at least 4' on both outsides of the cars. I always recommend an 18' door, as it provides good clearance yet doesn't mess up typical garage sizes too much.