Interesting question. In Masters thesis, you have to specify the improvements you have made in a field of study and why do they contribute to the field. 

Since you worked in 2 different fields, you can divide your thesis into 2 parts. First, discuss literature and problems you are trying to solve. Then find a common ground where your 2nd part can be helpful in your 1st field or vice versa. Finally, you can discuss literature and findings from your 2nd topic and conclude. 

Wish you the best in your thesis.