Oracle symmetrical one-way replication for cheap HA: unfashionable or technically unfeasible? We have two smallish unrelated databases on two big Oracle Enterprise servers. Instead of buying more licenses and servers for high-availability replicas, can we replicate database B to server A and database A to server B, as long as either server can handle the load for both databases if necessary?

Seems like there are several underlying questions:  


Which replication methods can replicate an active DB to a standby DB in both
directions between two servers with good failover?
How much lighter is a replicate-to-standby workload than a
production workload?
If people aren't doing this, are hidden gotchas stopping them or is
it just not in fashion?
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.