No problem. I created some test data to better illustrate my point 

create table #table (    customerID int,
                        salesAmt decimal(10,2),
                        orderDate date
                    )    
                    
insert into #table(customerID,    salesAmt,    orderDate)
values
(15652,20,'2017-01-01'),
(15652,20,'2017-02-01'),
(15652,20,'2017-03-01'),
(15652,20,'2017-04-01'),
(15652,20,'2017-05-01'),
(15652,20,'2017-06-01'),
(15652,20,'2017-07-01'),
(15652,20,'2017-08-01'),
(15652,20,'2017-09-01'),
(15652,50,'2017-10-01'),

(15675,20,'2017-01-01'),
(15675,20,'2017-02-01'),
(15675,20,'2017-03-01'),
(15675,20,'2017-04-01'),
(15675,20,'2017-05-01'),
(15675,20,'2017-06-01'),
(15675,20,'2017-07-01'),
(15675,20,'2017-08-01'),
(15675,20,'2017-09-01'),
(15675,70,'2017-10-01'),

(27059,20,'2017-01-01'),
(27059,20,'2017-02-01'),
(27059,20,'2017-03-01'),
(27059,20,'2017-04-01'),
(27059,20,'2017-05-01'),
(27059,20,'2017-06-01'),
(27059,20,'2017-07-01'),
(27059,20,'2017-08-01'),
(27059,20,'2017-09-01'),
(27059,80,'2017-10-01')

SELECT t.customerID, sum(t.salesAmt) as totalSalesAmountLastMonth,
   ca.totalSalesAmount as totalSalesAmountYTD
FROM  #table as t
   CROSS APPLY
   (
            SELECT customerID,
                    sum(salesAmt) as totalSalesAmount
            FROM  #table
            WHERE 
                    year(orderDate) = year(getdate())
            GROUP BY customerID
   ) as ca
   
WHERE 
   year(orderDate) = year(getdate()) and
   month(orderDate) = month(getdate())-1 and
   ca.customerID = t.customerID
   
GROUP
     BY t.customerID, ca.totalSalesAmount  

There is no need here to create anything within SSRS as the YTD field is here and ready to be pulled.