I have a client who's website I am redeveloping currently to run on SilverStripe. Within their website is a blog they run on WordPress, which we are not moving over to SilverStripe, because the company's blog master doesn't feel the SS Blog plugin is full featured enough, and doesn't have the budget to get us to upgrade it for him.
It's WordPress, so the blog will almost certainly need .htaccess to work.
But what everyone else has said: the .htaccess in the root directory is key. That should have rules to send the WP page requests to the WP entry point, and the SS page requests to the SS entry point.
Don't forget also that both CMSs have assets and files accessed within their modules, such as images and stylesheets. So long as both have clearly defined root directories it should be straight-forward.