1) make sure server allows more than 1 parked domain
2) change DomainB's nameserves to match those of DomainA.
3) log in to
http://www.DomainA.com/cpanel and click parked domain.
4) type
http://www.DomainB.com and press park.
Once the nameservers change over,
http://www.DomainB.com/forums-dir should link to the same thing
http://www.DomainA.com/forums-dir does, but will show up as
http://www.DomainB.com/forums-dir or
http://www.DomainB.com/forums-dir depending on what the user types. The same thing will show up for EITHER though, unless you put the forums directory in DomainB folder only.