Depending on which domain a visitor uses is it possible to have then sent to a temporary page.
so if they visit say www.oldsite1.com/*
they go to www.newsite.com/site1.html
and for www.newsite.com/*
they are taken to what ever page they wanted.
Website is rebranding and want new visitors that click old site links in searh engines or bookmarks to be taken to a new holding page that welcomes them and says that the new site will take over old and that they need to update their bookmarks.