concurrentmodphpis currently available as a USE flag for
dev-lang/php. It enables you to run two versions of PHP in parallel, loaded into your apache's mod_php.
With the advent of FastCGI/fpm in PHP-5.3.3 (very soon on a mirror near you), the preferred way to run multiple versions of PHP in parallel is CGI. Not only can you run several PHP versions independently, you also get proper script isolation (running each script as a different user, if you wish so) and thus enhanced security.
This is why I've decided to end support for USE=concurrentmodphp after php-5.3.3 and php-5.2.14. Versions after those will simply stop shipping the patches required to support this USE flag. Please use the time with PHP-5.3.3 to switch to fpm, if you need the functionality currently provided by concurrentmodphp.