Can you be the registered keeper on 2 cars?

You can be the registered keeper on as many cars as you like. And the same goes for being the main driver. There's no law which forbids you to do so just like you can be the owner of many houses.

However you can only use your no claims discount on one car only, unless you had more than one set of NCB (for people who have separate policies for each car they own). So it is better if you use your NCB on the car that's more expensive to insure. This will allow you to make the most savings.