Fetch Last 2 Bills

I want to fetch all bills (except cancelled bills) for a given month and fetch the previous bill date.

This is to find out the frequency of a vehicle visit to a workshop.

Sample Data

Division,ChasNo,BillDt,Status
D1,C1,01/01/2015,Null
D1,C1,15/01/2015,Null
D1,C1,23/02/2015,Null
D2,C1,26/02/2015,Null
D2,C2,02/01/2015,Null
D2,C2,16/01/2015,Null
D2,C2,21/02/2015,Null
D1,C3,24/01/2015,Cancelled

Result Expected

D1,C1,23/01/2015,15/01/2015
D2,C1,26/02/2015,Null
D2,C2,21/02/2015,16/01/2015

What I tried is

Declare @Mh smallint,@Yr smallint
Select @Mh=2,@Yr=2015

Select Division,ChasNo,BillDt Into #CMBills from Service_Bills Where Month(BillDt)=@Mh and Year(BillDt)=@Yr and Status Is Null

Select SB.Division,SB.ChasNo,CMB.BillDt as CurBillDt,Max(SB.BillDt) as PreBillDt,Datediff(day,Max(SB.BillDt),Max(CMB.BillDt)) as DiffDays from #CMBills CMB
Left Join Service_Bills SB On SB.Division=CMB.Division and SB.ChasNo=CMB.ChasNo and SB.Status Is Null and SB.BillDt<CMB.BillDt
Group By SB.Division,SB.ChasNo,CMB.BillDt

But I'm not getting details for all the bills I fetched.