The JCA adapter is thread safe and if you find any threading problems related to the JCA adapter, please report them to Oracle Support. Regarding OTMQ, I believe those libraries are thread safe as you can build a multi-threaded OTMQ server, but we don't support using JNI to call any of our libraries. Why would you want to use JNI? You can simply use the JCA adapter to enqueue/dequeue messages.
Oracle Tuxedo Chief Architect