If you have an existing WordPress based Job board website, it wouldn't be necessary to use JobRoller on another installation as you would essentially just replace the existing theme with JobRoller. Our
customer showcase has some great examples of how you can customize JobRoller to get a unique look, without the need for a separate theme for a home/landing page.
You'd only need to consider a separate installation if you really don't wish to lose the existing theme used, but even then, a visitor to the site would not necessarily realise you're using different themes if JobRoller was restyled to match the homepage.
If you wouldn't mind please clarifying, is
WP Job Post the exact name of the plugin you mention? I am aware of some other job related plugins, but they have different names, so I would like to make sure we're talking about the same one here. Thanks.