If there are two different Oracle UCP data sources with ONS enabled (even if they point at different databases/ONS servers and they are configured to use a different UCP pool name, so they should theoretically not affect each other):
1. ds1 obtains 1 connection - OK
2. ds2 tries to obtain 1 connection - it gets stuck for about 40 seconds and then fails with oracle.ucp.UniversalConnectionPoolException: All connections in the Universal Connection Pool are in use
3. subsequent attempts to obtain connections from ds1 and ds2: sometimes the exception above repeats itself intermittently, sometimes it works. I don't currently see any pattern when it works and when not.

The same exception sometimes occurs during XA recovery.