Double driveway gates are two gate leaves hung from opposite posts or piers, meeting in the middle. When both leaves open - inward or outward depending on your driveway - you get a full-width opening for vehicles.
Simple enough. But within that basic definition there's an enormous range of variation: how the timber is jointed, what species you use, whether the design is solid or slatted, arched or flat-topped, automated or manual. Each of those decisions affects how long the gates last and how they look a decade from now.
The most important thing to understand early: a double gate set isn't two gates. It's one pair, engineered to work together. Proportions, weight, hinge placement, how the leaves meet - all of it is calculated as a unit. That's why buying two gates separately and hoping they'll pair nicely rarely ends well.