> Moinmoin seems to allow sharing underlays
We found some minor problems with that (like e.g. links pointing to some
other farm wiki). So, maybe better don't use that except if you can live
with those problems.
> and user database
That should be no problem. Esp. if you will have some unified login for
your wiki farm and the same usernames/userids in all or most wikis of
it, it is recommended to do that.
> i.e Would it be upgraded/migrated properly
You have to be a bit careful with such a setup, moin has no special farm
E.g. if you run a migration script on the data dir, make sure you use
the user_dir like it was migrated by a single run of the migration
scripts. If you have to run the migration scripts for multiple wikis,
use a backup of the old user_dir so the migration script sees a old
user_dir and a old data_dir as input for every run you have to do.
> (if the underlay is used in
> two wikis, it means that the data would be migrated twice, isn't it?)
We don't migrate the underlay at all. It is usually thrown away on
upgrade. Being able to do that was the main reason why we invented
underlay some years ago.