A quick warning:
When you setup a logical standby database, you need to activate supplemental logging on the primary database.
This is done automatically when you build the data dictionary (by running the dbms_logstdby.build procedure).
Activating supplemental logging is however (I know now) a control file change and is thus not replicated to the other physical standby databases.
As a result, the logical standby will become (logical) corrupt when you perform a role switch between your primary and another physical standby database.
I learned this the hard way :(
Luckily it was during a proof of concept and not in a real production environment … .
Of course, AFTERWARDS, I found the following maa document which points out that you have to enable supplemental logging yourself on the other physical standby databases.
It still makes a good read though