I had a change in my travel plans and booked a tatkal end-end ticket (lucky that irctc was fast). I went to cancel the onward ticket at PRS counter and had to lose 25% of entire journey as cancellation charge (first train's departure was due in 36 hours) even though the next train ticket was in WL and departure was well after 3 days.
PRS guy told that I could cancel second leg alone but still will have to shell out 25% cancellation charges since the journey (first train) is starting within 48 hours. Dont know how true was that statement.