Dim rst As DAO.Recordset
    Dim rstSub As DAO.Recordset
    Dim dtLast As Date
    Dim dtPrev As Date
    
    Set rst = CurrentDb.OpenRecordset("SELECT [ИД Лицевого счета] as Id FROM таблица1", dbOpenForwardOnly)
    
    Do While Not rst.EOF
        Set rstSub = CurrentDb.OpenRecordset("SELECT [Показание] as Pokazanie FROM таблица2 WHERE [ИД Лицевого счета] = " & Nz(rst("Id"), 0), dbOpenSnapshot)
        
        If Not rstSub.EOF Then
            '-- последняя дата
            rstSub.MoveLast
            dtLast = rstSub("Показание")
            '-- предпоследняя дата
            rstSub.MovePrevious
            If Not rstSub.BOF Then
                dtPrev = rstSub("Показание")
            Else
                dtPrev = ????? '-- значение по умолчанию
            End If
            
            '-- делаем что-нибудь с полученными датами
            
        End If
        rstSub.Close
        rst.MoveNext
    Loop
    rst.Close

    Dim dtLast As Date
    Dim dtPrev As Date

    Dim lLast As Long
    Dim lPrev As Long