O2 Customer Services have contacted me about this. They said that they
are in the process of changing computer systems and that merging 2
accounts is not possible at the moment. There were unable to say when
the problem would be resolved - they could...
I tried adding a second phone to the account but I got a message which
said: This device is already registered to a My O2 account. Do I need to
un-register a phone, or delete one of the accounts first?