iTunes and iCloud have two different email addresses

You cannot merge Apple IDs. Ideally you would have only ever have used one. How many computers and devices are connected to these accounts? What do you have stored in the the cloud? I suppose that could all be downloaded to local storage, then you could switch iCloud IDs and upload it again. Purchases are tied to the iTunes account so no switching away from that unless it is generally unused, although you can potentially set up Family Sharing as a way of accessing the content of an old ID. If you have an iOS device managing apps purchased from multiple IDs is a headache you don't need.

tt2