The answer is “it depends.” A Notes application contains two kinds of notes: data notes and design notes. Data is, of course, data; design notes are the code and programming elements that control how the application behaves.
If the two databases have the same design, then you can copy the data notes from one database and paste them into the other (useful if you have just a few thousand notes or so). You can also write some basic code to copy the data notes from one DB to another if you have a larger volume of notes. This is a generalization; in some specific cases you could copy the data and still have it not work right depending on the design of the application and how the data was created.
If the two databases do not have the same design, then you really have two different applications that store and use their data in different ways. The odds of the two applications having compatible data structures are essentially zero. The data could be made to work in the second database but you’ll need a professional and skilled IBM Notes developer.