I am not sure, If we can go with below options in your case. I have not worked with such kind of scenario. But, I want to understand from other experts if we can go with below options.
Option 1 :
Multiple Components One Database (MCOD) i.e one database, several applications(ECC & BW) several schemas (Obviously ,Seperate Schema for ECC & Seperate Schema for BI)
Option 2:
Multiteneant DB i.e in one tenant ECC and in other tenant BI