Sometimes it will work if you just extract the content of all the floppies into one folder, and mount that as one floppy.
Odd, the manual says explicitly that you can't mount several floppies with imgmount, but also that you can swap them with Ctrl+F4 if you have them mounted, and that you can mount them with the boot command.
QBix has said in the (far) past that nobody has simply taken the time to implement it:
http://vogons.zetafleet.com/viewtopic.php?t=21213