Susan:
One way would be to break it down into sales per day and then multiply that figure by 60 days (or 60.8 using average days/month if you want to be particular).

-Number of days since April 1: Today() - Date(2012,3,31)
-Average sales per day: {Units Sold YTD} / [Number of Days]
-Sales for 2 months: [Average sales per day] * 60

The problem with this formula is that you have to edit it each April to change the calendar year. There's likely a way to work around that with some thought.