I want the two usb devices to always mount at the exact same mount point.
That's an easy one to solve. You want to use partition labels on each partition on your external hard drives. Labeling a partition results in the partition being mounted at /media/<label_name> no matter what linux system I've used. This is great for scripts since the mount point of the external partition doesn't depend on the order in which the external storage devices are connected; they always connect on /media/<label_name> not /media/<device_file>.
To set a partition label, you can use gparted for most supported filesystems:
Or you can use some command line tools:
Gparted can be found on any number of livecds but my current favorite partitioning livecd is partedmagic: