This is what I do, but it breaks down when legitimate email ends up in the spam folder in account B. You have to periodically log in to both accounts to make sure you don't lose important emails.
There's a way to fix the spam issue. In the source Gmail account, instead of activating forwarding the normal way, create a filter that will match everything (To: account@gmail.com), and make it forward and not mark as spam.
I do that with only one secondary email that generally wouldn't get any emails from real people, so in most cases, I'm covered. But you're right: that technic should come with a warning: you might lose your emails.
I suppose another way to go around might be to retrieve account B through POP or IMAP.
