Unify Contacts/people on outlook 2013 for all imap accounts

I have setup outlook 2013 for a friend and also an outlook.com (hotmail.com) account , as i needed it for office install and also to sync all his devices.

the hotmail.com , I want to be the default contacts , so when he chooses to send an email from any account , gmail, hotmail , then He can goto To: and his contact list appears, and he chooses the contacts to send
I want that people list to be on the hotmail.com account so all contacts sync across all devices

I have moved the hotmail.com contacts to be at the top of the calendar and when he sends from the hotmail account the list appears
BUT when he changes to the other email - on contacts he has to use the dropdown to select the hotmail contacts list

How can i do this with IMAP accounts please

I have outlook 2010 , so i can play with th esettings, so long as there are no differences between 2010 and 2013 as to how outlook handles the people