Andreas Kjellman has published an small, but very interesting bit of video on Channel 9.
You can read more in the Azure AD Connect documentation pages00https://channel9.msdn.com/Series/Azure-Active-Directory-Videos-Demos/Azure-Active-Directory-Connect-in-place-upgrade-from-legacy-tools/player
You can read more in the Azure AD Connect documentation pages
Additionally, I strongly suggest to have a look at the discussion/comments on the post.
Having a 2nd server is now supported. This is called a “staging server” and more information can be found here: https://azure.microsoft.com/documentation/articles/active-directory-aadconnectsync-operations/#staging-mode.
It is also possible to filter based on OUs. More information on filtering options can be found here: https://azure.microsoft.com/documentation/articles/active-directory-aadconnectsync-configure-filtering/.