It's an optical illusion. They're at least 1 plane length separated. The trailing pilot calls the break when separation is clear. This is a picture taken from a long lense so the depth of field is low giving the illusion they're very close. The trailing pilot always has his speed brakes open at this point. They pass overhead in perfect line abreast formation, then once they're far enough out in front of the crowd, the trailer pops the boards, gets separation, and calls the break. Gives a great impression of collision effect.