Why can't I book an equal-split flight that's already booked for that day?

With an equal split, the price depends on the number of people on board. If two separate bookings were made on the same day, each would divide the costs on its own side. Together, they would exceed the real cost of the flight and the pilot would profit, which the law prohibits. To keep the maths fair, only one booking is possible per flight and per date.

Travelling as a group?

Book all the seats together, in one go. The more of you there are on board, the lower the price per person.

Is the pilot flying morning and afternoon?

In that case, they publish two separate listings, one per departure (a button lets them duplicate the flight). Each departure is a separate flight, with its own costs and its own booking.

Is the date already taken?

Choose another date on the same listing, or opt for a flight offered with a fixed contribution, which does accept several independent bookings.