While it might not be possible to have the same account number for each line, it is possible that you can have the same log in details for your online account and view the details for each number via a drop down menu after you log in to your account.
As far as the credit check goes, in store it refers us to ring credit and risk, and they just check that there is 3 months 'clear billing', which basically means if you have paid your bills on time for the last 3 months, there should be no problem.
I suggest you pop into your local O2 store, mention you already have an O2 account, and they should just do the necessary for you.
Hope this helps