Can you have more than 1 derived part in a part.
And is it possible to select on which plane and location the derived part is to be?
I have a part with a derived part but now I want to add another part to it. is this possible?
For the first part of your question I would say yes.
Did you try it an not get what expected.
For the second part of your question I would say it is going to come in the same position it was originally modeled - but you can use Move body (actually I think I saw something in SDK or somewhere where you could translate/rotate on deriving).
Might be best to post a sample dataset to work through the questions. Sometimes what really answers the question is best solved with actual geometry.
you may find some information about changing the derived position through this link:
http://forums.autodesk.com/t5/Autodesk-Inventor/Placement-of-derived-features/td-p/2692827