Functionality and set up when Reciprocal customers is enabled

My business case is i would want to create reciprocal customers so as to track and record AR Transactions between the group companies. Now the question is as follows:

1. If we enable Reciprocal customers would it get applicable to the already existing customer accounts?

2. What is the sequence of set up once need to perform?

3. Functional behavior when we enter transaction for reciprocal customers?

Any direction on this would be very useful.

regards

Partha