SQL + VBA Populating Data in Cell Range
What you need is an ADODB.Command. It's the same query with different parameters, so you should use a parameterized query. Honestly, you should be using a parameterized query anyway. You have no idea what the user has put into those cells. This is a SQL Injection disaster waiting to happen. Let's say I put '; DROP DATABASE trackerfy2015 -- into cell E6. Boom. There goes your entire database. Poof. Gone. Up in smoke.



xkcd: Exploits of a Mom

So, about that parameterized query..



Dim cmd as New ADODB.Command

Const sql As String = &amp; _
    "SELECT sum(Hours)/80 " &amp; _
    "FROM payroll2015_rif " &amp; _
    "WHERE DepartmentCode = ? " &amp; _
        "AND payperiod = ? " &amp; _
        "AND paycode IN ('REG1', 'REG2');"

With cmd
    .CommandType = adCmdText
    .CommandText = sql
    .ActiveConnection = conn

    .Parameters.Append .CreateParameter(Name, Type, adParamInput, Size, Range("$E$6").Value)
    .Parameters.Append .CreateParameter(Name, Type, adParamInput, Size, Range("C20").Value)

    Set rs = .Execute
End With

' Transfer Result
Sheets(2).Range("$H20").CopyFromRecordset rs

' reset the value of the second parameter
cmd.Parameters(1).Value = Range("C21")

Set rs = cmd.Execute


Note that I didn't set all of the parameters for the CreateParameter function because I have no way of knowing the appropriate values. 

I need to mention something that you did very very right. Using Worksheet.CopyFromRecordSet is by and far the most efficient way to transfer data from a Recordset to a worksheet. Well done. You have no idea how often I see people loop through a recordset to populate a worksheet. I'm very happy to see you do it the right way. 



Some other notes:


You have unused variables wks and xrow. Put the first to good use by setting it to Sheets(2). This way if the destination sheet should ever need to change, you only have to do it in one place. Remove the other dead variable. 
Add an error handler so you can make sure the connection and recordset always get closed no matter what happens.