The first part of the solution to your problem involves writing custom udev rules for each usb device. Each device has a unique ID. Once the rule is written to mount a specific device to a specific mount point (/mnt/left or /mnt/right), you can then add an entry to your /etc/fstab for the devices.
The second part of the solution involves determining the unique usb port identifiers. I haven't the foggiest idea how to do that. If you can figure it out, the solution *may* involve the udev rules mentioned above.
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PS. Please pardon my manners. Welcome to LQ. This is a great forum for learning (just by reading threads posted by others), and for getting answers to your questions. But please use the link given above first. Most questions you may have are probably already answered on the web.