You can try these links, I haven't tried to set it up though:
http://tetro.net/misc/multilink.html
http://www.samag.com/print/
http://www.linux.com/howtos/Adv-Routing-HOWTO/lartc.rpdb.multiple-links.shtml
http://www.samag.com/documents/s=1824/sam0201h/0201h.htm
Please report back with your ...