There are three different Pentax hot shoes modules :
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First one (31045) is only a F5p connector.
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2nd one (31022) is a pass-through for the camera, where you can plug a F5P cord and still mount a flash (or another hot shoe module).
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last one (31046) (very similar) only have the top connectors. The bottom hosts a tripod thread.
As you can see, the two last ones are nearly identical...
So, with 2xAF540, you would need one 31022 with a 3m cable going to your 1st AF540's f5p plug, and then one 31045 on top of it with another cable going to your 2nd AF540.
The K20 user manual gives another example, page 150, but using AF360 (without the F5P plugs).