Can the output values of a function be added consecutively as in a loop.

I am using a function which gives me two outputs function [CDD,HDD] = UKMOequations(Month ,Day)

I get the output values for every specific month and date, is there a way I can add all of the output values (CDD & HDD) to get a sum of the values for every month and for the whole year as well, the values which I get are scalars.

I tried function [CDD,HDD] = UKMOequations(Month ,Day) + function [CDD,HDD] = UKMOequations(Month ,Day) for two different values of day and month but it didn`t work out. I was hoping if I could make a new function which just adds the values by calling this function in a loop, is this possible?