@sjt277 ,
The simplest approach is to use a separate USB-to-serial adapter for each ESC.
You can set up udev rules to map and distinguish the specific usb-to-serial adapters (using their unique serial numbers) to aliases in /dev/, such as /dev/esc0, /dev/esc1, etc.
Then run your testing using different USB devices to access each ESC (i am assuming you are using voxl-esc tools?).
For example, here is some relevant information: https://askubuntu.com/questions/49910/how-to-distinguish-between-identical-usb-to-serial-adapters
Does that help?
Alex