multi ERP source system, single DW load strategy

I am faced with the following scenario and was hoping this forum could provide some guidance:

My company recently went through a large ERP system conversion (in this case Oracle E Business Suite Financials). We currently have implemented a vendor specific "packaged" DW solution which we have customized to some extent to meet the requirements. The problem is that the conversion from the old system (11.5.7) to the new system (R12) did not take into account all BI specific requirements. Each OLTP system module was converted based on OLTP needs (for example 2 years of Supplier Accounts were converted, 10 years of AP/GL data was converted,etc etc). Our current warehouse only sources from the R12 system. The business now has a number of requirements that require data from the "old" system (for reporting, analytics, auditing)..etc.

Considering this, I need to find the best approach on how to meet these requirements. I want to try to not be at the mercy of the team that did the conversion (for example, supplier A in old OLTP system also exists in new OLTP system..or Fact AP data for last 2 years exists in both systems..but data older than 2 years exists only in old system).

I will undoubtedly have a Natural Key that identifies which system the data is being extracted from. What I would like to understand is some sort of methodolgy to handle this situation. For example, I have all my CUSTOMER dimensional data in my new system but for SUPPLIERS, its spread across both systems.

Is it even recommended to try to put everything into one DW? There is one option to load 2 separate DWs with 2 separate ETL loads, and then use my report tool to delegate the user queries to each system..but I would like to somehow have one DW for both current and legacy data. The other concern is with regard to change capture...I only need to load the legacy data once, but incrementally from the new system.

Any guidance, thoughts, suggestions are welcome.