I am able to see both drives simultaneously on another computer.
I tried using a powered hub as you suggest, but still only one drive appeared. I also tried connecting one drive at the same time as an external CD drive which uses two USB connexions because it draws more power than a single USB can supply. Both the Elements drive and the CD showed up. This suggests that the motherboard is capable of supplying enough power for two Elements drives, which only use a single USB each.
Also, the LEDs on both Elements drives were lit continuously, suggesting that they are getting power from the USB ports. The motherboard is an Asus Prime B550-Plus, which should be capable of providing enough power.
Since both drives show up on another computer, my current thinking is that the motherboard is not interpreting the ID data from the drives correctly and thinks they are the same device. I would be interested to see if anybody else has experienced this, but I think I need to pursue it with Asus.