This is actually not such a great idea if taken at face value. You would have to remember what cars you have in each garage, even if you can teleport to them. However, it can be a symbolic purchase, and you can still access all cars from one place. In that case, it actually does not matter you own a second garage. And since you're not limited by physical walls, you can just get more car lots in the one garage.
However, more car slots is a must. I realise this game is a sequel* to Underground 2, where you had five car slots, but in Most Wanted, you had 10, plus, if you did it right, you had an additional 15 cars (if you sold any, car slot went with it).
____
* No, it is not a reboot.