I should have tried it myself before saying confidently:

> To see the disk in another window, just type usbfat: there

In fact usbfat: doesn't quite do the right thing.

You can either fix /rc/bin/usbfat: by moving the lines
if (! test -e $disk)
usb/disk $opts || exit 'no disk'
to follow the line
if (! test -f /srv/usbfat.$user) {

or else just use
mount -c /srv/usbfat.$user /n/usb
to mount an already-mounted usb disk in another window.