good question, The reason to split is
- It doesnt overwrite the data which ll be replicated from EC to HCM as you see all data replicated will be compared with existing data and later replaced by it
- you are turning around the slave master now. EC becomes master and its like going live with new ERP system hence fresh transmission or Go live date is needed.
- most importantly do avoid retros as historical shouldnt be touched by EC and shouldnt be allowed to be mastered there. it make it a clear synch.
- also to reduce the amount of data to be replicated from EC to ECC
Hope i was able to clarify, let me know if you need more details.