Date parameter runs but want results from previous month to display as well

I have a report that runs by date from and to.  It normally includes a month of previous results.

If I run the report, I want to also give the previous month result.  Eg.  I run it for August 1st - 15th but I want results for a special field to display all results from July 1- 31st.

Eg.  Signs Repaired = Count(Signs Repaired) for August 1st - 15th = 30   Now I want last months result for all Signs Repaired in the month of July = 45.    The parameter start and to date is just for August 1st - 15th.  July's result is through SSRS expressions. 

I got as far as this...

=IIF(IsNothing(Parameters!EndDate.Value), "",DateAdd(DateInterval.Month,-1,Today()))