We actually have 4 instances of the system two of them are for non IT departments. Our maintenance team was looking to get a full blown school dude type building management system but was rebuffed, so they turned to ours a second choice in addition our publications department uses it as well.
The other two instances are the primary IT instance and a dev/test environment that really just for testing updates and major workflow changes to the system.
We do not bother trying to separate request types or asset types so we them we just purchased and set up additional instances. We have one DB server and just host the front ends on different VM's.