Youtube – Changing default YouTube account

You want this Unlink YouTube and Google Accounts

It is available via Account -> Manage Account -> Unlink YouTube and Google Accounts

However, if you signed up for YouTube after May, 2009, you can't unlink them.

You can unlink your old Google account from your YouTube account. In YouTube, under Account Settings->Manage Account, there should be an option "Unlink YouTube and Google Accounts." Once you unlink them, you should be able to link your new account with your YouTube account and multiple sign in will play nice.

YouTube accounts created after YouTube joined Google are considered part of your overall Google account. There is no real way of separating them (you couldn't separate your Gmail account from the rest of your account, per se). I have the option to unlink because I created my account before that, and Google sees my YouTube account as an account that just happens to be associated with a Google account.