This requirement comes in place if your application uses another resource, besides the TimerPool in the same transaction. To be able to commit properly, at least 1 of 2 resources need to be XA. If you are using the same database for the application and the timer service, you do not need to create 2 pools, just 2 jdbc resources that point to the same pool (or even tell the timer service via domain.xml settings which jdbc resource to use instead of the __TimerPool, but I personally think that it's more complicated).
HTH,
-marina
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