These two parts correspond to two different images, two senses that complete them or are parallel. However, even in the classical haiku, we can find some with only one image.
Caesura separates then the context from the action. See "a classical structure".
For example:
Shiragiku no me ni tate te miru chiri mo nashi
In
white chysanthemums
They
is no dust
that
is visible and draws attention
Bashô
Matsuo
Caesura is classicaly noted in the West with a semi-colon if it is at the end of a line, with a pivot word if it is an internal one.
The length of the parts are not frozen. We can found in classical Japanese haiku :
1. Caesura at the end of a line
5-12 : at the first line end
12-5 : at the end of the second line.
2. Internal caesura
8-9, 9-8, 7-10, 10-7