*** UPDATE: There is a new fully-automated version here:
Okay, RTFM says that this can be controlled on the host side. Before assigning an IP address to the second kindle, I can use a script mntusb:
#--- mntusb script
# U = usb number
# K = kindle number
sudo ifconfig usb$U down
sudo ifconfig usb$U hw ether ee:49:00:00:00:0$K
sudo ifconfig usb$U up
sudo ifconfig usb$U 10.10.$K.1 netmask 255.255.255.0
mkdir /mnt/kindle$K 2>/dev/null
sudo umount /mnt/kindle$K 2>/dev/null
sshfs /mnt/kindle$K email@example.com.$K.2:/
Each kindle plugged in gets sequential USB numbers starting at 0. I modified the usbnet config so that each kindle has a different 'K' number, which is used in its usbnet config IP address.
./mntusb 1 5
mounts my 5th kindle on usb1.
This script lets me ssh (or telnet) to all mounted kindles simultaneously, and access their mounted file systems. It could use a little more automation, and be called by the usbnet config script.
It is a good idea to unmount the kindle file system before ejecting the kindle from usb:
sudo umount /mnt/kindle$K
RTFM works! ;-)
P.S. I suppose I should have tested this offline so I did not need to edit my post so many times... ;-(