Managing personal Outlook.com account settings, security, and privacy
If you wish to "merge" them, I can think of two methods.
The first method has two parts to the process: the contacts and the messages. Moving the contacts is actually rather easy. Log into http://people.live.com/ using the old Microsoft ID, click Manage in the menu bar, and choose Export. This will create a CSV file you can save to your hard drive. Log out of the old Microsoft ID and log into http://people.live.com/ using the new one. Click Manage>Add people>Import from file. Browse to the CSV file you saved and click "Import contacts".
Moving messages is more difficult, but I think the easiest way is to POP them from the old account using the new account. Log into http://mail.live.com/ using the new Microsoft ID. Click the Options gear, then "More mail settings". Under "Managing your account", click "Sending/receiving email from other accounts". In the "You can receive mail from these accounts" section, click add an email account, then "Advanced options". You won't be able to enter yor old Microsoft ID account credentials on the first page because Outlook.com will tell you "It looks like you're trying to add another Outlook account" and suggest that you link the two accounts. On the "Advanced options" page, enter your old Hotmail address, the server name "pop3.live.com", use port 995 and check SSL, leave a copy on the server, then enter your old Microsoft ID credentials. Click Next and finish the wizard. You can set up a rule to move those messages to a separate folder if you wish.
If you have folders other than the Inbox, it will take manual effort to move those folders over. In the old account, create a new, temporary folder and move (not copy) to it everything in your Inbox. On the new account, do the same, if the old Inbox has been pulled over after what I described in the previous paragraph. Then in the old account, copy the contents of one of the non-Inbox folders to Inbox and allow the new account to pull those messages to its Inbox. When they have arrived, you can move them in the new account to a folder you create that's named the same as the source folder on the old account. Then delete the copy from the old Inbox. Do this step with every non-Inbox folder in the old account. Eventually, you'll have copied all of the old account's messages to the new account. WHen you're done, you can put the messages you saved in the temp folder back to Inbox.
If before setting up the POP access in the new account you move the Inbox to a temp folder on the old account, you can move all the non-Inbox folder contents first, and then finish up with the Inbox of the old account by putting the saved messages back in the Inbox. Note that I don't know how long this process will take because I don't know how frequently your new account will poll the old one and I don't know how you can initiate the polling. Send/Receive events don't seem to be controllable.
As far as your calendar goes, I don't know of any way to transfer that, since there is no export/import function that I can see.
The second method would be to install Windows Live Mail and use it to transfer the contents of one account to the other. This is probably the easier of the two methods and it should allow you to transfer the calendar as well.