Currently Being ModeratedNov 18, 2012 1:30 PM (in response to lindamoby)
You can't do it see http://support.apple.com/kb/HT4895
Currently Being ModeratedNov 18, 2012 3:08 PM (in response to Allan Eckert)
If your second account is actually an iCloud account with data in it, as opposed to a login for iTunes, iTunes in the Cloud and iTunes Match (which isn't actually anything to do with iCloud itself) you can sign in with it in System Preferences>Mail, Contacts and Calendars and its data will appear alongside the main account. (You can't use PhotoStream and a few other facilities with this secondary account, and you don't get 'push' email, only checking at intervals).
If the second ID is in fact only for iTunes just go on as you are: there is no conflict about doing this and you won't even notice as Keychain will log you in with the correct ID in each case. It's also better security not to have your iTunes login also your email address, given the number of complaints about hacked iTunes accounts.