Hi Frans,
It is good to keep 2 different project for your scenario, the reason would be
1. You can maintain 2 logical components ( one for bw and other for ECC) for one project, but the risk is you need to make a move for phase changes in both these system tracks, which is cumbersome, you cant expect this always both systems go same time for same phases.
2. BW and ERP logical components cant tie together in any solution with any valid business cases, until unless you have dependency like ( CRM goes to ERP) then you can tie these logical component together in a change project,
3, BW transports are quiet risk, its good to keep the separate charm project for bw.
Best regards
Jansi