@dfabulich, I am so sorry; that’s a really tricky spot…but there are some missing details here.
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Is FY also publicly viewable, as FX is?
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And are the users targeted (not just offended) by the FY mods on both FX and FY (i.e., FX+FY)?
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Do you have proof that the same two individuals are in fact the same two users?
If FY is also publicly viewable, if the users targeted by the FY mods are FX+FY (i.e., a part of both forum communities), and if you have indisputable proof that it is indeed the same Bob and Dave on both forums who are actually behaving as accused, then I think that you are clear to sanction the FY mods on FX (after a warning) as long as you have publicly documented proof that this is happening as alleged.
Otherwise, the FX+FY users who are affected by the FY mods on FY must either convince the FY admin to take action OR convince FX to become private. 
You have to be careful that you have all of the actual facts and are not assuming facts not in direct evidence.
And that the FX info discussed on FY is publicly available on FX is something to be considered; Bob and Dave are not in the wrong for merely screenshotting and posting what anyone can find. 