Start only external apache

To prevent the "internal" Apache instance from starting up when Caché starts, go to System Administration, Cponfiguration, Additional Settings, Startup and change the WebServer setting to false.

I'm not aware of any built-in ability for a Caché startup to start a different Apache instance.

Yes, е.g. for Windows:

for private Apache:

1. <cache-install-dir>\httpd\bin\httpd -k stop -n <instname>httpd

2. <cache-install-dir>\httpd\bin\httpd.exe -k uninstall -n <instname>httpd

for external Apache:

3. C:\Program Files\Apache\bin\httpd.exe -k install -n <instname>httpd

4. restart Caché