Here is an immediate solution:
1. There are 6 lower berths available under Sr. Citizen (SS) quota for 27th march.
2. to avail Lower Berth (LB) quota, a Sr. Citizen has to be travelling alone.
3. So you need to make two separate booking for your parents.
4....
more... If you opt for Sr. Citizen Quota concession by checking the check box in passenger information page, by default the passenger would be considered for LB quota (60 yrs for Male, 58 yrs for Female)
5. First you make your Dad's booking (Above 60 yrs age), then immediately follow it up with your Mom's booking (Above 45 yrs age).
6. If you are lucky enough, you will get the berths allotted in the same coach in same section.
7. If you are unlucky, you can always exchange one berth with any generous passenger.