When selecting an existing TwinWiki, then two cases are possible:
Focussing on CamelCase names (which hopefully are an allowed subset for all Wikis), the processing goes on as follows.
If the name is already on the target Wiki, then establish the link, by creating a clickable entry in a TwinPages list; if not, open a template in the target wiki with a defaulted backlink entry. Now the user can (further fill and) activate the template or not. If activated, then the bidirectional link is established.
These sketchy remarks are supposed to be only a memo, they need a lot of refinement/reorganization ...
I'm not sure it should be as user friendly (*), but it should certainly be user friendly (**). It's an interesting and powerful idea to open edit forms on foreign hosts. Maybe combined with UnifiedRecentChanges that might be doable.
(*) Stuff inside the site is more controllable by the site and more important to the site than anything outside the site. Thus, it will be much easier to make things inside the site easier to use than stuff outside.
(**) Whether it should is actually an important question. One way to regulate an organization is by modifying the PathOfLeastResistance. Is free trade better than trade barriers for building a nation's economy? Is open immigration better than xenophobia? This is the same thing. Building walls and bridges betweens sites impedes and improve flow of ideas and people, potentially changing things dramatically. This can be bad, but it can also be good. -- SunirShah
MediaWiki does TwinPages a little differently. There's markup in the page itself, like [[InterWiki:PageName]], to define a twin page (twin pages are mostly for interlanguage links in WikiPedia, so these are usually called "interlanguage links" there). There's configuration variables per-site that define whether InterWiki links are to be rendered inline or as OutOfLineLinks. The editor can force an inline link with a colon prefix: [[:InterWiki:PageName]]. --EvanProdromou