Is it possible to apply for a second masters degree in a different field of study? (different department)
It's always possible to apply, whether you will be accepted is a different matter. The best thing to do is to speak to the academic who is the programme director (or similar title), explain that you are interested in applying for the course and see what they say. They will know what the general rules are as well as being able to provide specific guidance. The fact that your current masters degree is in a related subject (different department, but probably the same faculty) means it should be considered relevant.

I applied for a MA in Classics &amp; Ancient History after getting a BSc in Computer Science, and then went back to do an MPhil in Computer Science, so it is possible to move subjects.