If memory serves correctly...
To do the "unread replies" thing, other than the link I mentioned, you'd need 2 MySQL databases and to split the forums. But, to have logins work on both forums, you'd have to make sure both setups point to the same members table in the SQL database (so one database would be the "primary" and the other would be the "secondary").
Then, just to top it all off, Unread Replies go by cookies, so you'd need to have a domain name for each forum as WELL so the cookies can be properly set.
It's do-able, but it's a pain and it means paying for two domain names. >_<