AD Tree or Forest??

I am managing 2 separate domains for a parent company that has 2 separate companies. Neither company will share resources. But I would like to manage it from a single point. What would be the best way to migrate these 2 NT domains to 2000 and AD? Should they be on two separate domains completely or two seperate forests or trees? They are connected by a dedicate line (256k) and would only be used for Internet access (one site has a T1 to the internet).

Any help would be appreciated.