If I am answering one question for multiple jurisdictions (for example, I am answering both for the United States and Canada), should I post one long answer or multiple shorter ones?
3 Answers
A single answer covering multiple jurisdictions
Coherence and reading
The best read is a single coherent answer covering multiple jurisdictions. It's best even to point to the differences in explaining them.
"In Norway, eating stinkefisk on a bus is illegal according to this law. There is no such law in German, but it might be considered a public nuisance under this law and be punishable. And in Somalia, nobody would dare to stop you from eating your Stinkefisk if you have a big enough gun on your lap.
One Research effort, one answer!
You started your research effort once and went through multiple jurisdictions. That is one answer's worth of research.
Preventing the appearance of Point farming
Another point is to prevent the appearance of just splitting the answer up to gain more points.
If your answer needs more than one answer post in characters...
If you really need more than 30k characters, then you either are too wordy and should consider cutting unneeded text, or the question is too broad.
When I address the situation in multiple jurisdictions, I write a single answer (example), using the applicable jurisdiction tags before each section.
Keeping these together in one answer allows me to present a comparative view. It would be difficult to do this across several answers.
As always in law, the answer is "it depends" :)
Personally, my gold standard for an answer is a quote from the law, backed by an explanation and a link to a court ruling (either direct or through a news source), ruling exactly like the explanation hinted it should be interpreted. So basically the law in question and a real life proof that your interpretation of the law is actually how a real life judge would interpret it.
That is my personal gold standard when I think an answer is perfect. I will vote positively when either a) I know the juristiction and know the answer to be correct, or b) don't know the juristiction, but see my gold standard convincing me it is correct.
With multiple juristictions in one answer, it is hard to do that. What if an answer contains 2 juristictions, one I know, one I don't know. Do I upvote that? Only if both are my personal gold standard? What if one is dead on and one is wrong? What if I upvote something, that is wrong, because they were convincing for one juristiction, but not for the other? How do I transport the message "this is unhelpful" for only parts of an answer?
So my personal wishlist is, if the answer has inherent logic, why some juristictions are all the same (lets say inside the EU because of EU guidelines, across US states because of the constitution or between AUS, CAD and NZ because they share common ancestry of their laws (I guess, no idea)) then by all means group them in one answer. Chances are, people knowing one legal system are well aware and good judges if that is actually the case and the logic grouping them is sound.
If however, you are answering for two wildly different legal systems that have nothing in common (let's say you were raised in Germany and moved to the US) then please make two posts. Especially if groups are represented differently, the SE system of judging quality fails this test. If your answer is spot on for the US, but dead wrong for Germany, 10 people from the US will say "yeah, that's the way it is" and the two downvotes from Germans will do nothing in comparison for a score of +8. Two seperate answers, one at +10 and one at -2 will make it abundantly clear, what is what.
