As I had said in the comments, crossposting is discouraged in the SE network. This is because each SE site has it's own group of experts. When a question is asked, those same experts look at it from their unique perspective to answer the question. Given this, asking the same question of two different sites might garner two completely different answers. Or it could be the same answer arrived at through two different paths. There's no real way to tell.
"But fbueckert," you say, "doesn't more people answering questions benefit the site?"
Good question. But it's based on a false premise: that everyone can understand (and apply) each answer equally. There's a reason the sites are separate, with established migration paths for content that doesn't fit in. When an answer is provided on any of these sites, assumptions are made as to who is asking, and viewing the question and answers.
For StackOverflow, it's generally assumed you're a programmer, and have somewhat of an IDE to go off of. SuperUser assumes you have a basic grasp of the technical aspects of a computer and can find your own way around. Cooking assumes you have an oven, and you've gotten past the burning water stage.
When answering on these sites, those are basic assumptions, and will color the answer accordingly. These core competencies are inherent in whichever site you go to; they're not going to teach you how to program, or how a computer works. The groundwork and skills for the area are assumed to already exist, and you're just refining and adding on to those skills.
A good example would involve the PS3. Say it overheats. Most gamers don't know the first thing about soldering, voltages, or identifying broken components. But they know how to follow instructions. So you tell them what they need to do to fix it, and they dutifully strip the PS3 down to it's motherboard, and chuck it into the oven. It doesn't really matter WHAT went wrong, and although a good answer would explain it in terms a gamer would understand, it's incidental to actually solving the problem. Does it really matter went blew up? Not really, so long as there is a solution to resolving the problem.
Now post that same question on Electronics. You're going to get directions on how to fix it, but chances are extremely good it's not going to involve baking your motherboard. They'll point out which bit is most likely broken, and a good way to bypass or replace the broken module. These directions will most likely mean absolutely nothing to any gamer that runs across it. "PCB? What's that?" "Soldering? I can't do that!"
Both answers, while completely valid, are not equally useful to the audience provided. Cross-posting the Electronics answer on Arqade won't help; it'll get downvoted into oblivion, even though it's just as correct as baking it.
Now. Back to cross-posting questions. Keeping in mind that perfectly valid answers are not equal, as demonstrated above. By posing the same question to two different groups, you're fragmenting the discussion. You've got two groups of people working at answering it, and they're not talking to each other. They probably don't even know that someone else is working on it. We also run into the issue of, "he said, she said". Remember, these groups are going to come at it from different perspectives, and unless you fully understand both viewpoints, there's going to be something lost in translation. And then we run into problems of, "But, X said Y! That completely contradicts your answer!". And then we're adding to the noise, because rather than getting your problem solved, we're bickering back and forth to figure out what's going on, and where the disconnect is.
The reason cross-posting is frowned upon is because it's the hallmark of a vague question. If it's broad enough to be equally applicable to multiple SE sites, you haven't refined the question enough. More thought needs to be invested to see which group would be best able to answer your question. You need to decide which site you think has the best chance of answering the question. If you don't manage to get an answer, feel free to delete and ask on another. That way, there's no duplication of effort, no bickering, and if you get an answer on another site, it will be applicable to those that view it.
