Create your main report dataset (dsMain for exmaple) with a query something like
SELECT * FROM myTable WHERE EmployeeID = @EmployeeID
I'm assuming you have the first parameter setup and working correctly to return your list EmployeeIDs. For this example we'll call this parameter EmployeeID/
Create another dataset (called for example, dsEmployeeName) with a query something like
SELECT EmployeeName from myEmployeeTable WHERE EmployeeID = @EmployeeID
In the second parameter (the one you want showing in a text box), leave the 'Available Values' blank and set the 'Default Values' to be from a query. Choose dsEmployeeName as the dataset and choose EmployeeName as the Value.
NOTE This will only work the first time round. If you choose another value from the drop down, the name will not be updated.
I don't know your exact requirements but if you can get both the ID and name together, why do you need two parameters, one of which does nothing really as it's not passed to the report?