Can both RAM and JDBC job stores be used together? I wasn't able to find an answer to this in the official documentation.
No, they couldnt be used together.
Usually a scheduler will be obtained by the StdSchedulerFactory. These factory uses a java.util.Properties class to determine the settings of the scheduler.
I.e. you could pass RAMJobstore as well as JDBCJobstore as value of the key org.quartz.jobStore.class, but in that case the key would be overloaded and only the last value will be visible.