the ejbTimer service is NOT clustered and supports no failover.
You have to configure the timer on each node with a different database!!!
Then the timer is started (configured) at one node and will be started at this node (even if it is alive).
Also it is possible to caonfigure the timer 'non persistent', in this case all nodes configured same. But the timers are gone after restart JBoss ;-(
You have to vote for the solution at http://jira.jboss.com/jira/browse/JBCLUSTER-153
if this is necessary for you.