Why can I not add my second outlook account as an alias to my main acct??

When outlook was first introduced, I secured an Outlook.com address that is similar (not the same) to my Hotmail acct. I wish to add this Outlook address as my alias, but when I attempt to add it, it says "This email address is already taken. Please try another. " I know it's taken... it's taken by ME !! I realise that linking accounts is no longer an option, but this seems ridiculous - Am I missing something ? I have also tried to do the reverse, i.e. add my Hotmail acct as an alias to the Outlook address, but I get the same result. Am I correct in assuming I have to DELETE the Outlook address I want to use before I add it as an alias? very counter intuitive if this is the case. Any help greatly appreciated.