If you use CMT the container will track which resource is involved in the transaction.
If the EJB is finished (mean that the business method return) the container will send a prepare to all XA resources and if successful a commit.
It is the responsibility of the container how to do this. AFAIK JBoss will send prepare/commit in that order the resources are got from the container( but this might change with different versions).
Also if you use different resources from an EJB, or a couple of EJB's, inside the same JBoss instance you do not need a JTS configuration. It will be more complex and might have performance drawbacks as it uses IIOP.
So a Tx is committed if you return from a method with a different Tx scope, let me shown an example:
Method1 (Tx.Required)
do DB stuff
-> M2 (Tx.NotSupported)
Tx1 suspended
-> M3 (Tx.Required)
Tx2
do DB stuff maybe different DB'S
return
<- (prepare)commit Tx2
<-
<-
(prepare)commit Tx1
The commits are done by the container outside your code (also outside the interceptors)
There is no configuration to control that.