Don't do the math, do the measuring. Park two cars beside each other the comfortable distance you want, open the doors, and measure. Also allow for the protection of the roof supports (columns) to minimize damages if one gets hit by a car.

Don't try to turn a porte cochere into a carport. It's a place to exit the car under shelter and to unload. The car should then move on to the garage.

The purpose of a port cochere is to have a covered place for the chauffeur to stop and unload passengers. Not cargo. Cargo was dealt with by the servants. At the rear servant’s entrance. And it wasn’t covered. A double is pretty rare. Because it was not needed. The car was paused, people got out, and the car moved to the garage. If you “need” a double, you’re either confusing what it’s function is, or you’re using it as a slight of hand for permitting a parking carport.

You’re basically building a garage without doors or walls on the side of your house. 90%-200% of the cost of the garage. Which is why you don’t see this done very often. That, and it needs a large property. And a large enough and tall enough house that it doesn’t look like a giant tumor on the side. Which, if you don’t get the proportions right, it will. Sometimes a covered walkway from the rear garage to house is the better option.

If you have the space, build it big enough to be a carport. It gives you more parking options :)