I only have one car but I don't use it for work (I'm withing walking distance so no need) so its not worth it for me to buy another car as a daily but many peeps do, all depends on the amount of mileage your doing, as in cost per mile?
You can get a fair few small Eco cars for less than £150 a month brand new. A friend of mine has just bought a Suzuki Splash 1.0, £20 road tax, 60 mpg etc etc. it does what it says on the tin, saves him driving his big, petrol guzzling Audi each day and overall it saves him money, even running the 2 cars.