you can book the ticket very well..
just on the ARP of first train ie A to B go to PRS and fill form like..
Train Name (first train's name) Date (first train journey's date)
Class..... Seats...
Station...
more... from "A" station to "C"
Boarding at "A" upto "B"
and ask the clerk to make Blank Paper Ticket, take the amount for journey from A to C on first day.
Go to PRS next day and fill form in general way as for 2nd train and give the form and blank paper ticket together, u'll have to pay just reservation charges on next day.