1. Create pac file like this:
   function FindProxyForURL(url, host)
   {
   return "PROXY PROXY1:3128;PROXY PROXY2:3128";
   }
2. Use the pac script as proxy in client hosts.
3. Run App/libcurl to connect to server side through the proxy configuration.

I expected the following

App is able to connect to server through the proxy1, if the proxy1 is N/A, then failover to proxy2

curl/libcurl version

7.49.1
[curl -V output perhaps?]

operating system

All

Note:
There was a email thread discussed about it. https://curl.haxx.se/mail/archive-2000-11/0032.html