According to “mod_fcgi is NOT a replacement for mod_fastcgi” and “Issues with mod_fcgid and multi-threaded FastCGI application”, mod_fcgid
I can't answer your exact questions about mod_fcgid, but Apache has moved on and the preferred method now in Apache 2.4 is to use mod_proxy_fcgi, http://httpd.apache.org/docs/trunk/mod/mod_proxy_fcgi.html.
It does handle multiple concurrent requests. Typically you might pass requests to the php-fpm handler.