Here a bit of a story.
I’ve created by mistake 2 iTunes accounts and I’ve bought iPhone apps under both of these accounts. A few days ago after I’ve (finally) upgraded my iPod Touch to OS4 I’ve lost all the apps installed on iPod except those ones for each I did create a backup on my PC. Trying to install back some apps I’ve seen that some of my apps were installed while other not. This was happen because, like I said some were bought under a different account. I will explain you in the next lines how to install all your apps, even those ones purchased under a different account.

The question was if is possible to cancel / delete / disable one of those two iTunes accounts so in the future I won’t repeat the same mistake of buying apps using both my accounts. Unfortunately there is no way to delete an iTunes account nor to merge multiples in one single account. The only thing you can do to assure that you’re not purchasing apps under the wrong account is to set to none the payment method from your account page in iTunes and of course to pay attention when purchasing the apps to be logged in under the right account.

Still there is a good part, authorizing your computer to work under both accounts will make possible to have on your iPod Touch / iPhone the applications bought under different iTunes accounts. So, if you want all the applications you’ve bought to be able to install on your iPod or iPhone you will have to login into your accounts, one by one, into iTunes, and then go authorize your computer for each.

Here the detailed steps:
1. connect your iPod Touch / iPhone to your computer
2. fire up your iTunes
3. go login into your accounts one by one (click in iTunes on iTunes Store from left menu then click on your account – top – right – under the search button)
4. after each login go Store / Authorize This Computer
5. after you’ve repeating authorizing your computer for each iTunes account go sync your apps but don’t forget to select all you have in the left side. Failing to select them all will make you loose what you already have installed on your gadget.

Now you should have all the bought apps on your device, no matter under which of your iTunes account you bought them :)

And another tip before you go – if you’d lost an app when syncing you can get it back at a no cost! If you know exactly the name of it go buy it again. You should get an message that you’ve already bought that app and asking you if you want it just to download it again. Tap yes and hooray, you’d get back your lost app. Now before you go look for your lost apps to recover them, make sure you are login under the same iTunes account from the moment you purchase them. Otherwise you won’t receive any message and you will just pay again for the same app!

Fortunately there is a solution, even a easy one:
1. turn on your computer and connect your iPod Touch / iPhone to it.
2. hold down both, the power and the home buttons (the iPod Touch / iPhone will power off after 10 seconds)
3. after theiPod Touch / iPhone powers off, release the power button (but keep holding down the home button)
4. in no more than a couple of seconds, you should hear that Windows sounds that means the iPod is being detected by your computer again.
5. release now the home button, and your iPod will be back alive, in DFU mode.
6. open iTunes (if is not already open) and restore your iPod Touch / iPhone firmware.

Your iPod Touch / iPhone will not be jailbroken now, but at least will be functional.