I can't see any reason that ebuild of net-misc/csync can't be installed at the same time with net-misc/owncloud-client (i've tried version 1.7.0). I have removed string !>=net-misc/owncloud-client-1.5.1 from the ebuild of =net-misc/csync-0.50.0 and successfully emerged it without any file collisions.
24 Feb 2014; Bernard Cafarelli <voyageur@gentoo.org> csync-0.50.0.ebuild: Also block newer owncloud-client, as it bundles ocsync
Thanks for the report! Indeed, this is a leftover from versions before 1.6.0: https://github.com/owncloud/mirall/blob/master/ChangeLog * Install libocsync to private library dir and use rpath to localize Now with older versions already gone from the tree, I droppped the blocker in csync