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There are two sides to this question

## Questions that are on-topic on more than one site

There were three principle I followed on Stack Exchange for questions that would be at home on more than one site

* They may be asked on either site. 
  
  This *does* lead to duplicates on the network which is moderately annoying. I would generally leave a comment if I know of a cross-site duplicate.

* Should not be asked "simultaneously" on more than one site.  
  
  Where the time frame after which it is reasonable to re-ask on another site depends on the activity of the hosting site. A day or two for a moderate activity site like physics.se, longer on lower activity sites.

* Let the user know if there is a more specialized site by leaving a comment on the lines of *"This is on-topic here, but you might find a more concentrated audience on [specialists-site]"*.

## Questions asked on the wrong site

In my opinion migration worked worse on Stack Overflow than in much of the rest of the network. Which is not to say that it ran really cleanly elsewhere, but for other sites it would be too strong to say it "never worked" or even "rarely worked". Partially a scale thing, I'm sure. 

The single biggest problem I saw with user votes to migrate from physics was users not really understanding what was on-topic on the target site. At the moderator level we used to ping the mods on the other site in the Teacher's Lounge for a sanity check, but that doesn't scale. We need a mechanism to (attempt to) insure that anyone casting such a vote knows what they are doing and cares about getting it right.

> **Proposal:** Voting to migrate has a rep/trust threshold on *both* the originating site and the target site.