If you would like to have more than one parent proxy you can add more cache_peer directives; one for each parent. Now you can define either weight or round-robin to control the way Squid will communicate with the proxies: while weight tells Squid to prefer one cache over another, round-robin tries to spread connections evenly among the defined caches.
First of all a simple example for two parent proxies:
cache_peer proxy.isp1.com parent 8080 0 no-query no-digest default
cache_peer proxy.isp2.com parent 8080 0 no-query no-digest
If you define more than one parent proxy you might want to set one as the default proxy, which is used as a last resort.
An example for weight:
cache_peer proxy.isp1.com parent 8080 0 no-query no-digest weight=1
cache_peer proxy.isp2.com parent 8080 0 no-query no-digest weight=2
In this example it is likely that the proxy from the second ISP will be favored over the first one.
And here an example for round-robin:
cache_peer proxy.isp1.com parent 8080 round-robin no-query
cache_peer proxy.isp2.com parent 8080 round-robin no-query
cache_peer proxy.isp3.com parent 8080 round-robin no-query
All connections to our proxy would be round-robined among these three caches. Because Squid treats all parents equally, it is currently not possible to define a weight here, e.g. to forward 50% of the requests to the first proxy and 25% to the second and third proxy respectively.