Normally cross-posting is not allowed (or discouraged) on SE network, however I think Area 51 has a different case. The questions here aim to define the scope of proposals and most of the time we don't know which proposal will succeed. If an example question helps to multiple proposals, I don't see any reason to avoid cross-posting.
Still, I need a confirmation (or refutation). Is cross-posting permitted here?
Yes, it is OK to cross post example questions on Area 51
It is not unusual to for there to be several similar proposals in Area 51 at the same time. If you decide to support all of them that is fine, and if you have 1 or 5 questions that are potentially a top 40 questions on all of the sites, you can cross post the questions at all the sites.
Personally I have cross posted on several proposals, I have also recycled my really good questions from proposals that did not launch to similar new proposals.
Only the most promising proposed site will actually launch. There is no risk that more then one of the proposals will launch.
