If I have my job code deployed on two JVMs with JdbcStore, does @DisallowConcurrentExecution ensure only one job is running across all JVMs?
Yes it will; this kind of attributes work even with multiple instances. The only important thing here is that this attributes it's based on the JobKey, not the code as described here.
Anyway, just as reminder, do not run Quartz on multiple server unless you have a deamon to keep your clock in sync. From Quartz guide:
Never run clustering on separate machines, unless their clocks are synchronized using some form of time-sync service (daemon) that runs very regularly (the clocks must be within a second of each other). See http://www.boulder.nist.gov/timefreq/service/its.htm if you are unfamiliar with how to do this.