Merge the queries following these steps:

1. Create a query of volunteers in 2008 (Query A)
Criteria:
Volunteer > Time Sheets > Date between 01/01/08 and 12/31/08
2. Create a query of volunteers in 2009 (Query B)
Criteria:
Volunteer > Time Sheets > Date between 01/01/09 and 12/31/09
3. Merge Query A and Query B with a merge operator of AND (Query C)

The results of Query C will be the 2008 volunteers who were also volunteers in 2009.

*Note: If the date range is greater than 2 years, each year would need to be in its own query and merged into the results of the previous merge.

OR

1. Create a dynamic constituent query of volunteers in 2008 (Query A)
Criteria:
Volunteer > Time Sheets > Date between 01/01/08 and 12/31/08
2. Save the query and close it
3. Open a new constituent, dynamic query
4. Go to Tools > Query Options > Record Processing
5. Mark the checkbox for "Select from query" and browse for Query A
6. On the Criteria tab, select:
Volunteer > Time Sheets > Date between 01/01/09 and 12/31/09
7. Click OK
8. Click Run Now
9. Save this query (Query B) 

The results of Query B will be the 2008 volunteers who were also volunteers in 2009.

*Note: If the date range is greater than 2 years, repeat these steps, each time including the previous query in a new query until the final query has the criteria for the last date range.