My preferred approach would be to do the pivoting of the data in the query for the dataset, like this:

select

sales_person_id,

JanJun2012 = sum(case when month(sale_date) <= 6 and year(sale_date) = 2012 then sale_amount else 0 end),

JanJun2013 = sum(case when month(sale_date) <= 6 and year(sale_date) = 2013 then sale_amount else 0 end)

from

sales_data

group by

sales_person_id

But this can also be done in SSRS by putting the following expressions in a scope that represents each sales person:

=Sum(IIf(Month(Fields!sale_date.Value) <= 6 And Year(Fields!sale_date.Value) = 2012, Fields!sale_amount.Value, 0))

=Sum(IIf(Month(Fields!sale_date.Value) <= 6 And Year(Fields!sale_date.Value) = 2013, Fields!sale_amount.Value, 0))