I think the answer to your question is that it's not possible. There are workarounds though.
Let me confirm what you're truly asking. Are you talking about 2 separate accounts on the iTunes Store? For example, you've bought music/videos with your account (husband@example.com) and your wife bought music/videos with her account (wife@example.com). Now, you guys want to use a single account, let's say yours, but still have access to music purchased on
wife@example.com. Right?
Well, if this is the case, you're out of luck since Apple has not yet allowed for merging of iTS accounts.
The workaround that may work for you is to authorize your iTunes app (on your Mac/PC) with both accounts. This will allow you to play music bought on both accounts on your computer. Not sure if this will work with iPods/iPhones, though.
Another idea would be to upgrade all of your DRM protected songs to iTunes Plus. This will allow your iDevices to play those songs, regardless of which account it was purchased under. Videos, however, will still be DRM'd and you're out of luck there.
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NOTE - if you're not talking about merging separate iTS accounts and are talking about separate MacOS/Windows accounts, then it's much easier. Just import the songs from one library into the other. Then use the function to check for duplicates.