Load testing your API by using just ab is not enough. However, I think it's a great tool to give you a basic idea how your site is performant.
If you want to use the ab command in to test multiple API endpoints, with different data, all at the same time in background, you need to use "nohup" command. It runs any command even when you close the terminal.
I wrote a simple script that automates the whole process, feel free to use it: http://blog.ikvasnica.com/entry/load-test-multiple-api-endpoints-concurrently-use-this-simple-shell-script