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This answer to the question Whats the protocol if two satellites collide? shows the image below, and I don't understand the shape.
I would have expected two "jets" of debris, each projecting roughly in the direction of one of the satellites. But this one has what looks like four "jets" as two sets of back-to-back pairs.
How did a collision produce this pattern?
Your expectation is correct - this is two "jets", one for each craft, offset by around half an orbit so they appear to form a cross.
The visualisation is from this Wikipedia article on the 2009 collision of Kosmos-2251 and Iridium-33. It shows the spread of debris 50 minutes after the original collision, viewing the antipodal point of the original collision. Iridium-33 and Kosmos-2251 were both in ~100 minute orbits.
When they collided, some pieces of debris were knocked into higher energy and some into to lower energy orbits. This explains the longitudinal spread of the two debris fields.
The two larger, more dispersed "jets" to the left/top are in fact the pieces of debris in higher orbits that have not yet completed a half-orbit to reach the antipode.
