You can link them in your My O2 by logging in and then selecting "Bring in other products".
This will only allow you to see both accounts under one log in though and not as one bill and the operation cannot be reversed, so once done you cannot separate the 2 accounts ever again.
I have no affiliation whatsoever with O2 or any subsidiary companies. Comments posted are entirely of my own opinion.
This is not Customer Service so we are unable to help with account specific issues.Currently using:
Pixel 7a (O2 & Lyca), One Plus 6 (Sfr), iPhone 12 Pro Max (Vodafone)