You then have to edit the apache2 ports.conf and virtual host confs to listen only on 127.0.0.1.
Do the same for apache2-light, but use your external IP address.
Set the reverse proxy from apache2-light to send only php requests to 127.0.0.1.
Disable mod_php on apache2-light.
Enable APC on php.
With 1GB of memory and a heavy PHP ( apache2 uses 50 MB with APC enabled), I set my apache2 to 15 clients and my apache2-light to 50. Also I can now enable Keep Alive on the light.
My next goal is to have the light use worker, though the heavy must always use prefork to work well with PHP.
Any one interested in a full write up and will try this out?
Both of these were more complex than really needed as the Debian 6 apache2 script is so well written. It makes it so just copying that init.d script to another name lets it use a completely different configuration.