There are circumstances for which you can request the second date. Eg.reglious.
I think there was a situation one year when all the candidates from one primary school had to sit the second date due to a school trip!
Yes, that was last year, when a large contingent from one primary all had to sit on the back-up date due a clash with their year 6 residential.
On the CSSE website near to the closing date this year, you would at one point have seen the message along the lines of, certain venues were already at capacity, try requesting others. This later changed to something like, no places left (not sure whether at all or just one gender or the other) on main date, but you could book straight off for the reserve date (i.e.without having to claim religious objection). I wonder if this means they may have to offer a second 'sick' date this year if people were willing to go for the main date but had to accept the later one - I'm not sure what normally happens to those who decline the main date on religious grounds then fall ill on the reserve date?