In our Scrum teams we use a backlog, which mostly contains functional topics, but also sometimes contains technical topics. The advantage of having 1 backlog is that it becomes easy to choose the topics for the next sprint, but I have some questions:

• First, to me it seems more logical to have a separate technical backlog, where developers themselves can add pure technical items, like: we could improve performance in this method, this class lacks some technical documentation, ... By having one backlog, all developers always have to pass via the product owner to have their topics added to the backlog, which seems additional, unnecessary work for the product owner.
• Second, if you have a product owner that only focuses on the pure-functional items, the pure-technical items (like missing technical documentation, code that erodes and should be refactored, classes that always give problems during debugging because they don't have a stable foundation and should be refactored, ...) always end up at the end of the list because "they don't serve the customer directly". By having a separate technical backlog, and time reserved in every sprint for these pure technical items, we can improve the applications functionally, but also keep them healthy inside.

What is the best approach? One backlog or two?