You cannot - This is not supported as of now, and I don't think there could be any alternate solution until Google offers an improvement to its Multiple-sign-on capability.
Google's multiple sign-on capability helps you manage more than one of your accounts simultaneously. Signing-in to a different account when you're already signed-in, would link both your accounts (on the client side). And on each additional sign-in, each of the accounts are linked to one other. Signing-out from one account would implicitly mean that you sign-out from all your accounts (similar to a domino effect).
Google must support unlinking of two signed-in accounts for you being able to selectively signing-out of an account, but retaining the signed-in status of other accounts.