List Unrelated Degree on Application
I would say to list it, for a couple of reasons:


It does show that you are hardworking, 2 concurrent Masters degrees completed and completed to a great standard.
Practically, it further demonstrates the abilities necessary to research, synthesise information, draw valid conclusions from a wide range of sources.


Also, you put in the effort, why not 'show off' your good work. By steering your Literature Masters in a biological direction with your topic of tropical diseases, you have also shown how this long-standing and serious issue (tropical diseases) have been portrayed to the non-scientific community.