Sum non-unique values over months

Hello,

I not sure what should be the subject of my question, but here is what I am struggling with doing.

I created two measures, [New Customers] and [Returning  Customers] that count users. These users are shown in the following visualization. The chart shows those measures by months (month names are in Hebrew):

Now, I have to create a new mesure that sums up values over months.  I.e., finds the total number of New and reutrning users.

For returning users, the measure should provide: 253+376+596+ ... +482+702. The new meausre has to count the same user twice if they returned over two month.  

How can I do that? How can I split a meausre by month and then sum over months?

Thanks,

Erez