The current informal "policy" has been that it is allowed, within limits.
The main rationale is that there is no way to draw a consistent, meaningful boundary between the scopes of (some of) the different sites. This is especially true here, due to the intrinsically interdisciplinary nature of quantum information and quantum computation.
"Within limits" means that there should some rationale to ask the question on the different sites.
This also reflects what has already been mentioned and discussed on the general meta post.
In particular, asking the same question on different sites (hopefully tailoring it to the different audiences) allows to reach different communities and gain different perspectives on the problem, as argued e.g. here.
Moreover, there also seems to be a general consensus that different sites should stand on their own, and if a question is on-topic on multiple sites, it should be allowed to stand in all of them, see e.g. the answers to What to do with cross-site duplicates?. Quoting Roberto Cartano's answer there,
You can't assume that all users will be on all sites. Each site has to stand on its own merits and you have to treat each site as a separate community.
If a question is inappropriate on one site, users now have alternate places to go with it. It's a nice convenience if moderators choose to transfer it for them. If it turns out to be a duplicate, let the other community handle it with the means already in place. If the first moderator happens to spot it as a cross-site duplicate, it can simply be closed and the original poster can decide how to rework the question if they want to try it on another site.
In the edge case where the question is appropriate on more than one site, leave it on both sites and let the users of each community benefit from the information.
Now, I am not saying that cross-posting does not have its disadvantages. Some that I can think of are
- It duplicates maintenance efforts as questions on different sites live independent lives.
- If the OP does not link the other question on the other site, and someone else doesn't catch it, it potentially leads to a waste of time as people might give essentially equivalent answers without being aware that someone already did on another site.
- We lose all the functionality provided by linked and related questions tabs.
On the other hand, what is gained is mostly different perspectives on the same questions (which can be very interesting and valuable information).
But regardless of pros and cons, I'm not even sure that we could devise a sensible policy to forbid cross-posting that doesn't involve software changes in the site. Sure, the situation in which the same person asks the same exact question on multiple sites can be handled: just close all but the first posts (assuming that we even know that the question was cross-posted). But say Alice cross-posts a question on physics.SE and quantumcomputing.SE at the same time, the post on physics.SE is closed, and then a year later Bob, unaware of the previous post, asks a very similar question on physics.SE. Should this now be treated as a duplicate of a question that was answered on a different site? I think this sort of position would be hard to maintain. Also, different sites should stand on their own, not rely on discussions happening on other sites. See also this relevant discussion on vi.meta.SE and this relevant post on the stackoverflow blog.
This said, it might be useful to have guidelines in place as to how cross-posting should happen. In particular, we definitely want askers to
- Provide a link to the duplicate (or even just related) posts on the other sites.
- Tailor the question to each site as much as possible.