Your users still need to login separately to each section. Integration with phpBB does not include a login session for both at the same time. I tried awhile back, unsuccessfully, to even allow dual logins at the home page of the site. Logging in to one section kicked the login for the other section. From what I was told here, it's most likely due to the extra security added to Zen Cart logins. I'm sure someone with much more encryption experience could probably rewrite the login cookies and everything else that goes along, but I couldn't figure it out.