Is it better to have one website with two languages or two websites with different languages in terms of SEO?

There is no big SEO advantage using either top level domains, sub-domains, or sub-directories. This is well covered in the question: How should I structure my URLs for both SEO and localization?

The biggest differences are going to be:

• Top level domains and sub-domains allow you to move the hosting near the users by getting servers that are in the correct country. This can improve response times that make a difference both for usability and SEO.
• Sub-domains and sub-directories allow you to share cookies between the different languages. This makes tracking and login seamless.
• It can be difficult to obtain your domain name for each top level country domain. Expect to spend a great deal of time and money if you try to do so.