Oracle symmetrical one-way replication for cheap HA: unfashionable or technically unfeasible?
These days, Data Guard is included with Enterprise Edition.If you have enough spare capacity on each of the servers, I believe they can work as a physical standby for each other.

Logical and possibly bi-directional replication is more complex, and you would have to consider the types of objects and replication in the solution. Most solutions does not easily support the replication of DML etc. 

Note that Data Guard configured as physical standby might still consume a considerable amount of resources and depending on the configuration, may impact on production performance on both servers.