Don't know if you implemented anything or not but this will almost certainly work for what you are trying to do. I've used many on different projects and with different devices without any issues. You want the top one that has all DB9 connectors. I'm also an Elan G! dealer.

[Link: bb-elec.com]

The traffic from the controllers would be isolated from each other. The return traffic would go to both but that wouldn't be any issue because either the controller would know how to respond to the info it receives, or it would just ignore it.

Shane