You can insert a table in the Note and have two columns with different notes.
There are some good reasons it is one note per branch and this is to do with the exports:
- Word: a note is the paragraphs following a header (topic text)
- Web: a note is the content of a web page whose name is the topic text
- Project: a note is a task (topic text) note
I support Ary make it two subtopics. This allows you identify the purpose of the two notes.
You can also create invisible callouts, with no line or fill colour and no text, linked to each other and each with a note to create the illusion that the parent subtopic has multiple notes.
See the example below. In the first subtopic I've left the outlines of the callouts so you can see the callout locations but in the second I've made them completely blank: