Thanks for your reply.
I tested this in en.wikipedia (see here: http://en.wikipedia.org/w/index.php?title=Wikipedia:Sandbox&oldid=832643... )
I could cause a edit-conflict while editing two different sections.
Only in my own MediaWiki-Installs I could find this behavior :(
For hints to configure my wikis like WP-Servers I would be grateful.
Regards, Jan