Those that noted Venus' and/or Mars' lack of a magnetosphere are correct. Both of those worlds have a solid (or nearly so) core, preventing any magnetic feild caused by moving metallics (a dynamo) from reaching beyond their surfaces. Because of this both worlds have lost mind-blowingly large masses of atmosphere to solar winds. The reason Venus has such a thick atmosphere, especially when compared to Mars, is that Venus' surface is being boiled adding to the atmoshperic volume. Currently this process is matching the pace of loss. If the two bodies formed with each other's mass, then the large Mars would still be a cold planet with a thin atmosphere and the small Venus would still be an inferno with a thick atmosphere. This would be for the same reasons that their temperature and atmospheric qualities are currently. To get an habitable Venus or Mars (looking long term achievement and habitability), both planets would need a large moon that would knead their cores starting a dynamo (to create a magnetosphere protecting the atmosphere from solar winds, and the surface from lethal levels of radiation), stabalizing their wobble, and breaking Venus' near tidal lock with Sol. tl;dr moons have more effect on planets than location