Comment 1 for bug 890219

What you want to achieve is out of scope of apache2-suexec-custom. The filename in /etc/apache2/suexec is the name of the run user of apache2, i.e. whatever is specified as 'User' in /etc/apache2/apache2.conf. Or put it differently, that's the user suexec changes from, while SuexecUserGroup specifies the user suexec changes to.

This is described in the suexec man page, but I guess the description could be more clear.