Hi Gunasekaran,
as Dawood already said your best bet is to start by reading the available documents. The bidirectional replication of business objects between CRM and ECC is quite complex (at least for some objects). So please be sure to understand the implications of activating the replication. In particular a scenario as the one you described requires careful planning beforehand. For example you need to check the number ranges for all objects you want to replicate and make sure there are no collisions.
Best,
Christian