Hi there,
I have done this a while ago. The usb stick should be partitioned as sdb1,sdb2,etc.
The multiboot option depends on which image you are using (key1=sda1,key2=sdb1, etc.). I personaly have pgi in flash, an external usb hdd, a usb stick with swf, aaf, pkt, barracuda, both connected to a usb hub with external power suply.
Then after you have connected everything and turned the box on, through telnet do a fdisk /dev/sda and sdb. then check if the box has swapped the names of sda and sdb with each other (in my case it had changed the stick to sda and usb hdd to sdb).
This was solved by booting to SWF (enigma2) through PGI multiboot option, installing it, then changing the boot option through SWF MENU/PLUGINS/MULTIBOOT (ie renaming of different boot options, assigning multiboot images to particular keys and so on).
And remember, that the multiboot keys on your remote should be hold pressed longer than 5 seconds (10-12 secs).
Good luck