We have an existing C++ application that we are going to gradually replace with a new Java-based system. Until we have completely reimplemented everything in Java we expect the
There is an example on Laurent Schneider's blog of using the DBMS_XA package inside Oracle to permit multiple sessions to work in the same transaction. So it would be possible to have Java and C++ sessions participating in the same transaction without needing any sort of additional coordinator.
Alternately, you might consider using Workspace Manager. That was originally designed to support extremely long-running transactions (i.e. manipulating lots of spatial data for a proposed development). Essentially, you can create a workspace, which in your case would be roughly equivalent to a named transaction. Both the Java and C++ code could enter that workspace (from separate sessions) and both could manipulate and commit data in that workspace. When the transaction was complete, you could then merge the workspace to the LIVE workspace, which is equivalent to doing a commit in a normal transaction.
On the other hand, I would strongly agree with your initial assessment that coordinating transactions between processes is very likely to be a bad idea from a performance, stability, simplicity, and maintenance standpoint. On the other hand, it may well be a legitimate business requirement depending on how the C++ code is going to be retired (i.e. whether it is possible to replace code in such a way that transactions can be either exclusively Java or exclusively C++)