Log into your My O2 http://www.o2.co.uk/myo2 and select the option to "Bring in other products".Follow the instructions on screen to log in to the other account and then merge the two together.
Hope that helps.
Veritas Numquam Perit
Bring in other products is not an option?
Do you not see this image?
If not then you need to contact O2. Guide: Coronavirus Community Help and Support
Also be aware that once merged they can't be split again.