When I try to add a second character to my szene (Poser 4 Pro Pack) there are three possibilities.
1.) Change Figure (which makes my first one vanish)
2.) Create new figure (which makes my second figure takes on
many features of the first one, like body build for once)
or
3.) Import Poser document (which doesn't opens characterfiles at all)

Any help in that matter? Any experiences?

I think you want to use option 2. What you've described is cross-talk and you'll need to load a null figure. I don't have any links but I'm sure you'll find enough info if you do a search on "cross talk" here.

Thank you! Will go looking... But do you imply I can't put two different characters together? Only the originals? That would be too sad...

You can put two diffrent characters together, but as PabloS said, you need to load a "null figure" to shield them from cross talk.

Thank you again! I loaded a "Null" and it works perfectly!!!