I've done some work for a broker before, but I've never done one with 2 new drivers on 1 car. I think you probably won't pay more. 2 people can't drive the car at the same time, so having 1 new driver or 2 new drivers won't make a difference, 1 driver can drive 100% of the time already, it makes no difference. Ultimately it will be cheaper than having 2 cars though.
There's some small things that can lower your premiums, like make sure to take restricted driver option and maybe up your excess. Also using market price is usually cheaper than agreed value (even if you agree less than market value usually), and if you lock it in a garage at night and (usually) don't park on the street during the day it can go lower (insurers tend to ask where/how the car is stored during day and night).
The best thing you can do is ask an insurer, they will be happy to help you. Give em a ring or check their website, they will probably have to give you a whole quote to tell you.