benchmark multiple URLs using ab
ab can't do this, but this github project can. takes a file with a list of URLs.
With Apache Benchmark (AB) it's not possible to hit multiple URL's from a single instance.
You have to use multiple test or alternative is using tool like JMeter to achieve your requirement.
Note: Apache Benchmark doesn't test things like image processing, ajax calls, etc. which you might be expecting in real world scenario.
You can use parallel command to run multiple ab commands simultaneously. More details with Simon Holywell