Not at all. You did Wordpress installation through one-click install. So you can set up phpBB too,even thats easier than Wordpress!!
Yep,you can either on two different subdirectories or two subdomains or even one on a subdomain and other one a subdirectory. Just needs different destination.
Although you can create two different forums,i think its better creating a single forum. Because you can have different(as many as you want) sections(that act as different forum just like 'Curious about DreamHost','Beginners forum' and others on this forum) on a single forum. You can restrict people from using any section of your forum through different criteria. Such as limiting by number of posts the user posted,user groups(by creating yourself) etc etc. So no need to create two different forums. Just create a single forum that will also be easier for you to manage/administrate. As of phpBB is on the DreamHosts 1-click install,just proceed with it if you're not experienced on self-managed installation or don't bother to do so or if you're not intended to install other application.
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