Yes, you can have rules in both htaccess files but it's going to be tricky as wordpress has specific ways that it will expect URLs to look.
In the wordpress htaccess file, above any wordpress related rules, add:
RewriteCond %{THE_REQUEST} \ /+wp/careers
RewriteRule ^ /careers [L,R]
then in the htaccess file in the document root add:
RewriteRule ^careers$ /wp/careers [L]
But again, depending on what wordpress expects the "careers" URL to look like, this may not solve your problem.