syntax for using a parameter in a expression

I am new to SSRS reporting and looking for some help in the correct syntax for using a parameter in an table expression
I have built a dataset query that pulls the deduction codes used for two different payroll runs and I need to compare the deduction amounts between the two pay periods. My dataset query looks like this:

SELECT
prpmdded.prpd_warr
,prpmdded.prpd_emp
,prpmdded.prpd_ded
,prpmdded.prpd_empe_amt
,prpmdded.prpd_empr_amt
,prpmdded.prpd_empr_taken
,prpmdded.prpd_amt_taken
FROM
prpmdded
WHERE
prpmdded.prpd_warr LIKE @prpd_warr OR prpmdded.prpd_warr LIKE @prpd_warr2

My parameters pull the proper results from the database, For example the results for employee 118 and deduction code 2110 looks like
prpd_warr prpd_emp prpd-ded prdp_empe_amt
142013 118 2110 58.39
152013 118 2110 60.05

What I am trying to do on the report side is to have a table that looks like:

Emp Ded Code AmtPP14 AmtPP15
118 2110 =(Fields!prpd_empe_amt.Value) for PP14 =(Fields!prpd_empe_amt.Value) for PP15
which would show as
118 2110 58.39 60.05

I believe that you can use a paramater as a filter in an expression but I cannot seem to get the syntax correct. Of if I am wrong and this cannot be done this way what would be the best way to approach it.
Thanks in advance

Jim