I feel like I've tried just about everything to join two different meta data packages and have had no luck. Is this even possible or am I just doing it incorrectly? Is there a guide anywhere on the web that documents this process?
So many variables -- so little time.
What version of Cognos?
CQM or DQM?
Then, of course,... What are you actually trying to do? Are you trying to add two packages to a report? Are you combining two packages into a module and using the module in the report? ...something else?
As for the questions is this possible and where's the documentation, the answers are yes, it's called linking.
It is documented here:
Segmented and linked projects
https://www.ibm.com/support/knowledgecenter/SSEP7J_11.1.0/com.ibm.swg.ba.cognos.ug_fm.doc/c_segmenting_and_linking_projects.html
This is, of course, assuming that you want to link two FM models into a parent model.
You need to be aware of what you want to accomplish and what metadata exists and whether the dimensions are conformed or not etc. The definition of the problem, as currently stated, does not suggest that as firm a grasp of the requirement
I'm guessing you are trying to join queries in report studio but just saying you've had no luck doesn't provide much in the way of what you have tried and what errors you get.
I brought this issue up in a Ask Me Anything about Cognos session with IBM. If a report has two queries from two different packages right now you cannot join them in the report. To me this makes this option useless. Apparently IBM is considering it.
If this is what I think it is, I do this successfully all the time. And there are multiple ways to do it. There's just not enough information in this thread to enable me to be certain enough about the question to provide an answer.