Multiple non-query Parameter Values

I am semi-technical, new to Reporting Services, and have a question I can't find the answer to in books.

I am setting parameters for a field, where the user can go in and select the following options:

Expense Accounts - Value = 22
Revenue Accounts - Value = 30
Both - Value = 22, 30

Which is how I set these values in the parameter box, with the query statement as:

where account_type in (@account_type)

Unfortunately when selecting the Both option, this method does not work. How can I go about setting the parameters to list either, or both?

Any help greatly appreciated.
Laurie