If you cross post to multiple forums here, the moderators will lock the extra threads. Do it too often and you'll likely get moderated yourself.
It's frowned upon because it's viewed as a kind of spamming, and because it splits the focus of the discussion. All you'd likely get is two or more separate conversations going at once, often with overlapping advice.
So just choose the forum that you feel is the best fit for your question and post it once. If the mods think it should properly go somewhere else, they'll move it. If you feel you've posted something in the wrong place, or if you accidentally double-post, just hit the "report" button and ask them to fix it for you.
BTW, don't try to "bump" your threads either, at least not right away. Many users rely on the the zero-reply list to find threads to respond to, and you'll just remove yourself from it unnecessarily. Unanswered threads automatically get bumped to the top of that list several times over the first three days anyway.