I like to use a 'holster' style bag, eg the Lowepro toploader type bag, which can carry one camera with 70-200mm lens. If I'm shooting with two cameras, one with a 70-200 and another camera with a wider lens, I keep one camera in the bag and the other with the strap around my neck or over my shoulder and the camera in my hands. It is easy to quickly switch between them when needed - taking the camera in the bag out and replacing it with the other.
The main advantage is that it is much easier to move around, run, climb fences, be jostled by crowds etc than it is when having two cameras bouncing around by your side. Maybe also slightly noticeable than having two cameras on show. The bag is also handy if you want to carry a flashgun in it too.