There is no specific document on how to convert an existing one-way replication to a two way replication. The following servers an overview guideline. Please refer to Shareplex Administrator Guide Chapter titled "Preparing for Shareplex Replication" section titled "Establishing peer-to-peer replication" which will also provide valuable details while elaborating on some of the points mentioned here.
Here are the broader steps:
1. Make sure that the target tables are in sync with the source tables. Compare, copy or other means may be used. If there are too many tables then a fresh resync of target may be a better option.
2. Create conflict resolution routines that will be deployed on both source and target databases. See "Shareplex for Oracle Administrator’s Guide" for details.
3. Create conflict resolution file on both source and target.
4. Stop Post on target and activate a config on the target that replicates data to the source. Once the activation finishes, then start Post. From this point the original source can be termed as trusted source and the erstwhile target can be termed as secondary.
5. Enable triggers and constraints on secondary that were disabled as a part of one way replication. The triggers need to be enabled for users other than Shareplex. Run splex_add_trigger.sql script to achieve this. Also run this script on the trusted source to enable triggers for users other than Shareplex.
Once the setup is complete, applications/users can be directed to both nodes as needed.
It is recommend to test this procedure in a QA environment.