They should have merged on their own if one was a UA account and one was a CO account. There must have been enough differences in them to keep the system from seeing them as the same person.

I would first try the method on this page

Merge MileagePlus Accounts

and if not after seven days contact customer service for help

http://www.united.com/web/en-US/cont...s/default.aspx

and let them see what the issue was and combine the accounts.

Wouldn't expect a quick response from them though.