It's a separate Business Model - that assumes you are going to logically relate things differently. The bigger question is why the need for the second BMM?
You could have two ways:
1. Use a Oracle BI MUDE to MERGE your RPDs.
get the new RPD with the new changes, and after it deploy in Enterprise Manager)
2. It's not a good practice, however, if there is a few changes, you could open your RPD in online mode, and copy manually, the new changes.
In your case shortcuts may not work !!
I would suggest to development instead of shortcuts. dev work time is shorter than fixing issues with copy paste!
Short answer: You shouldn't.
Longer answer: You really shouldn't precisely *because* of the relationships and content levels!