It's a tricky question. Some modules there will be a requirement to pass, achieve any grade, in order to obtain your joint honours degree.
However, assuming you'll pass every module, your creative writing can potentially have a positive weighting to your overall mark.
You will need to see your individual modules and how much credit each is worth, but I will give an example below:
Let's assume your English Lit modules have a total credit value of 45 - and your Creative Writing modules come to 75 (They must total 120, I believe, each year). Then statistically speaking your creative writing modules will have a larger effect on your final grade than your English Lit, because those individual modules are accounting for more of your final grade.
In the attached images I have included an excel sheet I use to work out my grades. The example includes one module with a credit value of 15. Once you have all of your weighted average scores, you find out the average of these using =AVERAGE(), which shows your current grade. If you have a module worth 30 credits, for the Excel sheet it is easiest to duplicate it, as if the module is 15 credits twice.