I think this is basically how we're doing it.
<Proxy balancer://app1balancer>
</Proxy>
<Proxy balancer://app2balancer>
</Proxy>
<VirtualHost *:80>
ProxyPass /app1/images
!
ProxyPass /app1/stylesheets
!
#continue with other
static files that should be served by apache
ProxyPass /app2/images
!
ProxyPass /app2/stylesheets
!
#continue with other
static files that should be served by apache
Alias /app1/images
/path/to/public/images
Alias /app1/stylesheets
/path/to/public/stylesheets
#continue with aliases for static content
Alias /app2/images
/path/to/public/images
Alias /app2/stylesheets
/path/to/public/stylesheets
#continue with
aliases for static content
ProxyPass /app1
balancer://app1balancer
ProxyPass /app2 balancer://app1balancer
</VirtualHost>