When I was collecting two years worth of documentary evidence about me and my partner living at the same address, I got various opinions from different forums and home office hotline. With my personal experience you need 20 letters (almost one document for each month for the 2 year period) with both names addresses (partner and yours), but in case if you do not have everything with both names together, you can produce two separate files, one file for your documents addressing only your name and another file with your partner’s documents.