Margaret,
This is absolutely fine - your mum can be the main driver on a second insurance policy without any problems and you can be the second named driver on that policy as well. However if your mum becomes the main driver, she must be using the car more than you do, otherwise it would be classified as car insurance fronting which is basically putting an older and more experienced driver on a car insurance policy along with a newer/younger driver to bring the premiums down. If that is the case and you are found to be driving more than your mum, then your insurance will be void and any claims put forward rejected by the insurer. So if you'll be the occasional driver and your mum the more frequent driver, then there's nothing to worry about. It might be worth looking at multi car insurance discounts as well.
