We're here again?
A good question is very unlikely to be identical on multiple sites
I've discussed the no-cross-post debates over the months and I never could understand what was wrong with posting an identical question on two sites when it was perfect for both sites as-is.
It is pure hubris to contend that a given question is perfect for multiple sites. Each site has its own set of expectations of what a good question is. Each site has its own focus to what types of topics it covers. When the identical wording is posted on multiple sites, it is quite likely to be too broad or unclear, assuming that it is on topic on each site.
Optimize for the single answer
One should consider precisely the single answer that one wants to the question and post it on the appropriate site. This is the criteria. Not if each community will find it interesting or not (I look on reddit for interesting things), not if it is "perfectly" worded.
I follow a number of tags on different stack exchanges through email updates and filtered questions. Let me decide what interesting.
diverse answers can be a problem
Getting a diverse set of answers points to another likely problem with the question - that there is no one answer. Getting a range of answers means that there is less likely an authoritative answer on the subject (The halting problem - or - the fallacy of “real questions have answers” touches on this).
On wasting time
You have a problem. You are looking for how to solve it. You ask on Foo.SE and Bar.SE about how to solve it. Someone on Foo has an answer that solves it - great. But now people laboring over the problem on Bar.SE are wasting their time because the problem is already solved.
Posting to multiple sites wastes the time of the people answering it on the other site when there is already an answer to the problem on another site.
on improving the question
It is unlikely that this question is a 'perfect' one. Please accept that. Given this, it is then up to the community to tease out the details that are pertinent to that community and modify it so that it does present a question focused on their specialty. When this is done on multiple sites, it wastes the time of multiple communities to tease out the pertinent details.
on migrating and mods
One approach to this is for the question to be migrated to the other site where it is gathering the most attention or would likely get the best answers. I will point out that this is an area of debate - that one should try to cultivate each Stack Exchange independently (if there is a question on one SE, one should try to make it a good question for that SE rather than sending it to another one).
However, migration takes time. If you have gained answers on both sites and it is migrated (something that occasionally happens with cross postings between Programmers.SE and Stack Overflow), it requires either a mod to move it (SO to P.SE) or 5 close votes (P.SE to SO), and then 5 more close votes (dup) and then a merge (mod handles a flag). This wastes a lot of time that could have been better spent elsewhere.
on people finding the answer
A key part of Stack Exchange's value proposition is that it is the place to go to for the answer. One spot. The 'one' is important. Stack Exchange exists as an alternative to the forums for trying to get answers - having to search through pages and pages and sites upon sites trying to find the answer.
By scattering answers across the network to a single question one wastes the time of the people trying to find the answer and this in turn reduces the value of Stack Exchange's value proposition and diminishes the brand.
Don't post the same question on multiple sites. It wastes the time of people answering the question, of people trying to gather the answers into one place, of the people trying to find the question... and is unlikely written to be a good question on each site in the first place.