When i enter two words in the search (also in Automation) then it happens that i get results which contain only the first word.
Bug or Feature? Do other clients behave same? Perhaps a failure of some file-offering clients?

Generally on Gnutella AND is the rule, but some clients like limewire have a "feature" that will give you a hit on keywords that are not in the file name, user defined words that will return a hit.
You need to filter any returned results on your end if you want to make sure your results use AND only.

Care to give a brief description on how to for example search for: The Ark
without getting hits on Linkin Park and many others, with and without "ark" in their file names?

if it's something special, write a simple perl script to use as a filter, there are example scripts supplied