Insurance agent here:
If you have 1 car, and add a second, it will almost always raise your overall bill UNLESS the multi-car discount offsets the price of the 2nd vehicle. This is generally only the case if you first car is REALLY expensive to insure, and the 2nd car is really really cheap. That multi-car discount is usually a % off. My company gives about 20% off. So if your first vehicle is running you $100 a month for full coverage, it's now $80, but unless your 2nd vehicle is an old ass beater with liability only, you probably aren't getting it as cheap as $20 a month, that would be the extreme low-end of insurance pricing around this area of the country.
So, 95% of the time, adding a second vehicle is never actually cheaper. It can be in rare cases, but it almost never is. You're insuring two cars instead of one, of course its going to be more. Just like insuring 4 cars will almost always cost more than insuring 2 or 3. Each car is individually priced, the only thing that might offset some of the cost are additional discounts you may qualify for with more than 1 vehicle.
Also, we get the argument all the time that "well, I can only drive one vehicle at a time, why does the fact that I have 4 cars cost more than 1". A lot can happen to a vehicle while you aren't driving it. It can get stolen, it can get hailed on, someone can sideswipe it, your wife/kid/relative can use it. Chances are if you are a single person with 3 or 4 cars, there's going to be an occasional time that a relative or friend is going to borrow one. There's definitely risk there, even if there is only 1 owner and multiple vehicles.
If you carry full coverage on a vehicle, the price isn't completely dependent on the vehicle itself, a lot of the price is based upon the liability side of the policy, which is based on your age & driving record.
So say for instance, a brand new BMW may cost 3X as much as a random new Ford Taurus or whatever, the insurance isn't 3X as much because a lot of the cost of the policy is the liability, which would be pretty much the same on both. So in this scenario, say the liability side of the policy runs $50 a month for both, then the full coverage on the Focus is maybe another $30, while the super expensive Mercedes full coverage is 3X as much at $90, but that makes the overall prices of the policies $80 vs $140. The Mercedes ends up being more, but it's not a straight ratio of the value of the vehicles by any means.
Your liability portion of your policy is priced based on your age and driving record, the full coverage(comp & collision) is based off of the type of car, and BOTH are affected by the zip code/state/county where you live.