a transaction can across more than one persistence unit?

[javatwo]

start a userTransaction,

update in one persistence unit/data source A
update in another persisence unit/data source B

commit the userTransation;

My question is : do the two updates belong to the same transaction? Thanks.
dave

[javatwo]

I got exception when trying updates tables via two persistence units in one transaction:

ERROR [JDBCExceptionReporter] Lock wait timeout exceeded; try restarting transaction.

Is it allowed? Thanks!

[javatwo]

two merges are ok. but when creating one entity A, merge entity B, A references B (foreign key), Lock wait timeout error !