Sorry but sadly - and absurdly - you can't merge iTunes accounts. Blame Apple. This is a very common problem and many in the Support Community had hoped Apple would fix this with the new iCloud launch - but the people there don't seem to understand the intense frustrations it causes. When users change email addressses, or susbscribed to different packages like MobileMe or iCloud, they tend to create different log-ins. Worse, over time they sometimes forget old log-ins, making impossible to access features like apps purchased in the iTunes store. Whatever you do keep a note of your different account names and passwords.
I have this same problem, with at least 5 iTunes accounts I have found so far. What I've done is delete and re-download all of my apps--this was simple enough, if frustrating. Luckily, I only use free apps, so no money out here, just time re-configuring apps.
Next, I used iTunes Match to 're-brand' all of my music under the current iTunes account (the one I use for iCloud and my most recent iTunes purhases and will continue to use for iTunes purchases going forward). This is also cost-free, but tedious. I believe this functionality is meant to allow you to show 'owenership' of burned-in CD's, but it works just as well for content in your library that was purchased with a previous iTunes account. Each matched song is deleted on my computer and re-downloaded from iCloud. The new version is 'branded' with my current iTunes account, and will then be seamlessly available across my devices. Those songs in my library that are 'uploaded' (not matched in iCloud) are less seamlessly availbable, but this is getting better as time goes by and iMatch functionality improves. I only have one song I had to re-purchase, and it was a very old purchase that was DRM-protected from the old times. So, total cost for this was $1.99 (+tax) and lots of time on the couch 're-branding' music.
This brings us to video content, which is the crux of my problem. Currently, video content is not 'matched' in iCloud the same way music is, so the 're-branding' trick doesn't work. All music videos, movies, and TV Shows not purchased with the iTunes account I am currently using (and will use from this time forward--it is my iCloud account) are not available in iCloud, and thus not available on my iPhone and Apple TV. In the case of music videos, I don't mind re-puchasing them on a budgetary basis over time. Many of them are old anyway and newer versions would be welcome.
As to movies, the real problem for me is digital copies from DVD/Blu-Ray packs where the download codes have expired. These will never be available to me in iCloud unless and until I am able to merge accounts, or 're-brand' content as I did with music above. But the number here is so low that it really isn't a problem. Especially with 1080p content and iTunes Movie Extras, it is likely I would want to purchase some of these movies directly from the iTunes Store with my current account, although I am still on the fence as to Apple TV vs. Blu-Ray purchase for movies. I especially enjoy commentaries (which I am not seeing as available on iTunes movies), and very often the special features differ between physical packages and Apple TV purchases.
The main problem for me is TV Shows: this comprises the vast majority of my iTunes purchases in the realm of video content. I really wish there was some way to 're-brand' my 100's of gigs of TV content so that it would all be available in iCloud on my Apple TV without having always to use Home Sharing and stream from my laptop. But as with the movies, I think I am just going to have to take a 'wait and see' approach. After all, the problems I had with iTunes Match in the beginning are getter better and better as time goes by, and I expect that the same will apply with video content in iCloud. After all, many people have 'burned in' movies from legally purchased DVD's to the iTunes library, the same as with CD's, so 're-branding' ought to be allowed once the legal issues are resolved with the studios, the same as Apple did with the recording industry copyright holders. It might mean Apple needs to generate some more revenue to satisfy the movie and TV barons, but I would pay an increased iCloud fee to be able to get my movies and TV shows tied to my current iTunes account, and thus available to me in iCloud across my devices.
Hope springs eternal!
I just experience the problem : my appleID account forced me to create a username and when I try to give one (old or new), I got that stupid answer « there is already a username associated with that email address… you may not associate more that one username per email » STUPID !
More stupid yet : it was my login for… « Apple support » so it was impossible to ask for help without creating another new account.
Hopefully I remembered later the other password I used with that support service. But now, what am I supposed to do ?
Apple become stupid ! Thinking there is only one way to go IS stupid ! (Easy for newbies but nonsense to reality.)