Connecting two BW systems to the same ECC system (or any other SAP system) is (normally) very easily accomplished. You simply create a source system connection to the ECC system (or other SAP system) in RSA1 on each BW system. Simple.
However, where this becomes difficult is if one of your BW systems was built as a system copy of the other BW system. Let's say you want your Training BW system to point to the same ECC system as the QA training system, and your Training system is a copy of your QA system.
When you try to restore the Source System connection in RSA1, the process will prompt you to delete the existing connection on the Source System side. That works fine for the BW system that you are setting up at that time, however it severs the connection to the existing BW system. I could go into great detail on why this occurs, but I am sure most of you are looking for a solution and want it quick.
So, here's the solution (all steps are performed on the new BW system unless noted otherwise):
1. Execute SM59 and delete the RFC connection to the Source ECC system (or other SAP system).
2. Execute RSA1 and delete the Source System definition
3. Execute RSA1 and create a new Source System definition, utilizing a different USERID for the RFC than what was previously used, since that user ID will already exist in the Source System and is tied to the existing BW connection. Keep in mind you will have to have the ECC client opened for changes in order to activate the connection.
Now both the original BW system and the newly copied BW system can connect to the same ECC system.
Hope this helps...
