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I can't find the stats or rules anywhere in the codex. I've looked in the armory and the front section where the weapon stats are listed. Would I consider it to be double the assault rate of a regular Shuriken Catapult?
You use the stats for the regular Shuriken Catapult... It just counts as Twin-Linked, which is detailed in the main rulebook.
4thEd page 30, bottom of the right column, "Twin-Linked Weapons". Also see the examples on page 31.
As You will see, it does not increase the amount of shots You get, merely increasing the odds of a hit.
"The meat things seek to destroy ourselves. They hunt us as we hunt them but they are weak and uncertain. The inferior flesh will be entirely destroyed, all biomatter consumed. Ourselves will fight to the last, all weak flesh must be devoured."