Can I add two SEO plugins on one WordPress site?

You can technically install as many of any plugins as you like on WordPress. That being said, you should step back and examine the functionality of each plugin to make sure they won't create conflict with each other. There are different types of plugins that aid in SEO which pair well together.

 Widely regarded as one of, if not the best SEO plugin for WordPress, and not without its reason. Yoast SEO offers an all-in-one solution to improve your WordPress site's SEO performance.

Yes you can add two plugins but it is not recommendable.