Communication between multiple RT systems

We currently have 2 RT installations. One is 4.0.4 and the other is
3.8.8 (which is on schedule to upgrade very soon). From time to time,
a ticket is submitted to one system that should have been sent to the
other and sometimes a task will get passed from one to another as the
task could have multiple parts.

I know I have seen dome information about this before, but have not
found what I’m looking for yet. If someone could point me to some
documentation, I would greatly appreciate it.

Thanks in advance!