To do this, you would use a two page form letter type mail merge main
document with the text and merge fields set up for each side of the card in
that document.

Then, with your data source which must be in the form of a table in a Word
document open as the active document, you would run the following macro.
This will ask you how many cards there are across and how many down, and
then it will create a new data source with the records duplicated in the
correct order so that when you attach this new data source to the form
letter type mail merge main document, and execute the merge, the required
information will appear on each side of the card.

Dim i As Long, j As Long, k As Long, l As Long, n As Long, m As Long
Dim dtable As Table
Dim arec As Range
Dim dsource As String

dsource = ActiveDocument.FullName
Set dtable = ActiveDocument.Tables(1)

GoThere:
m = InputBox("Enter the number of cards across (1 or 2)", "Duplex Labels")
If Not IsNumeric(m) Then
   GoTo GoThere
ElseIf m < 1 Or m > 2 Then
   GoTo GoThere
End If

GoHere:
k = InputBox("Enter the number of rows of cards on the sheet.", "Duplex
Labels")
If Not IsNumeric(k) Then
   GoTo GoHere
ElseIf m < 1 Then
   GoTo GoHere
End If

dsource = Left(dsource, InStr(dsource, ".") - 1) & " - Duplex for " & m &
"x" & k & ".doc"
If m = 1 Then
   i = (dtable.Rows.Count - 1) Mod k
   If i > 0 Then
       For j = 1 To k - i
           dtable.Rows.Add
       Next j
   End If
   For l = dtable.Rows.Count - (k - 1) To k + 1 Step -k
       For j = 1 To k
           dtable.Rows.Add (dtable.Rows(l))
       Next j
   Next l
   For j = 1 To k
       dtable.Rows.Add
   Next j
   For n = 1 To k
       For i = dtable.Rows.Count + 1 - n To k + 2 - n Step -2 * k
           For j = 1 To dtable.Columns.Count
               Set arec = dtable.Cell(i - k, j).Range
               arec.End = arec.End - 1
               dtable.Cell(i, j).Range = arec
           Next j
       Next i
   Next n
Else
   k = 2 * k
   i = (dtable.Rows.Count - 1) Mod k
   If i > 0 Then
       For j = 1 To k - i
           dtable.Rows.Add
       Next j
   End If
   For l = dtable.Rows.Count - (k - 1) To k + 1 Step -k
       For j = 1 To k
           dtable.Rows.Add (dtable.Rows(l))
       Next j
   Next l
   For j = 1 To k
       dtable.Rows.Add
   Next j
   For n = 1 To k
       If n Mod 2 = 1 Then
           l = -1
       Else
           l = 1
       End If
       For i = dtable.Rows.Count + 1 - n To (2 * k + 2 - n) Step -2 * k
           For j = 1 To dtable.Columns.Count
               Set arec = dtable.Cell(i - k + l, j).Range
               arec.End = arec.End - 1
               dtable.Cell(i, j).Range = arec
           Next j
       Next i
   Next n
End If
ActiveDocument.SaveAs dsource

See http://www.gmayor.com/duplex_merge_data.htm