If you have a single web server and a single database server that are active and then a single web server and single database server that are passive, NO you do not need to have 2 licenses.
If you want to have a load balanced setup where you have 2 or more servers in a web farm, you will need to purchase an additional license for the additional servers in the web farm. That being said, you can still have your active/passive setup with a web farm environment as well without additional need for another license.
Keep in mind the above are talking about 2 different things:
- active and passive servers
- load balancing