I am working in a single farm with two distinct web applications. Each web application's content needs to be isolated from the other. Should I provision two Search Service Applications or use a single Search SA and rely on security trimming, separate content source indexing, and proper scopes?
I am already planning on unique content access accounts for crawling each web application.
Two search service applications make sense when you need to make sure, that "a query from site A to return results from site B, even if a poorly created scope included both content sources". You will then use search service application proxies to assign each web application their own search service application. This will give you a good level of security- however, you should be aware that security trimming might not work as aspected when users permissions change frequently and/or are extremly complex.
