I know that people do, but the NHS website advises against it unless on advice from a medical professional:
www.nhs.uk/common-health-questions/childrens-health/can-i-give-my-child-paracetamol-and-ibuprofen-at-the-same-time/
Can I give my child paracetamol and ibuprofen at the same time?
No, don't give your child paracetamol and ibuprofen at the same time, unless advised to by a healthcare professional.
If you give them one of these medicines and they're still distressed before the next dose is due, you could try the other medicine instead.
Call your GP or NHS 111 if you've tried both medicines and they haven't helped.
Don't keep switching between doses of paracetamol and ibuprofen unless advised to by a healthcare professional.
I think this advice is there because you could potentially mask a more serious problem with overuse of painkillers. If one or other isn't helping then there could be something going on that requires medical attention.