You can be the main driver on the 2 cars if you want to. Actually you can be the main driver and policy holder
of as many cars as you want. However if you have no claims bonus, the discount can only be applied to just one car. Therefore for the second car, it will be treated as if you have zero no claims bonus as you will have used it all on the first car.
Most insurance companies will give you discounts on the second car if you insure both cars with them though. The best thing to do is use your no claims bonus on the car which is more expensive to insure to bring down the cost of the premiums.