Of the two examples you posted, the second (about Bleach) was posted before this site existed, so Scifi was the correct place to post it. The first was posted after we entered public beta though.
In my opinion, either of those questions would have worked on either site. They're within our scope here. They're also at least somewhat in scope on Scifi (as neither has been closed). So you're allowed to ask on either site. There's a meta post on Scifi regarding this, but the short version is that you're allowed to ask on whichever site you feel is likely to give the best answer. Cross-posting the same question to multiple sites is not allowed though. The same is true for overlap between this site and Movies & TV, Gaming, and any other network sites we may happen to share some of our scope with (now or in the future).
So, if you think your question is best answered by science fiction fans, ask on Scifi. If you think your question is best answered by anime fans, ask here.
With that said, I suspect that for most questions about anime/manga, especially for less popular series than Bleach or Dragon Ball, this site is going to be the best option. Scifi has a few knowledgeable users about popular anime series, but here nearly every experienced user will have seen these shows. In fact, in my opinion, the big reason for creating this site in the first place was that there's a lot of anime/manga questions which require specific expertise not common among the larger communities already present on SE. The questions were mostly technically already on topic somewhere on SE, but the expertise was very fragmented. By gathering them in one place, we've created a community which is more specialized on these topics, and hence typically more knowledgeable.
To make this comparison explicit, Scifi seems to have about 100 questions that I could find easily on anime/manga, while we have nearly 3000.