parseLookup(DD1)+parseLookup(DD2)... should work, but make sure parseLookup() is typed in with correct case (capital L) - use rather formula builder when creating formulas.
reg. filter - you likely hit the mentioned bug. what exact form version are you on?
That's the problem: I currently don't have access to the central administration but will look it up as soon as possible. Thank you very much so far, will get back to you as soon as I have any further information regarding the version or when I got it working.
Version is 188.8.131.52, the current version is 184.108.40.206.
In version 220.127.116.11 they changed it, so I definetely have to install new version. Thanks a lot for your help, will get back to you.