http://members.shaw.ca/jiuwan/AddEffect3x.jpg
Thats a pic of a RDM/NIN with Excalibur dual wielding it and a joyeuse. If you notice in the pic the add effect went off 3x in a row, which means the add effect procs on the offhand weapon also, in this case a joyeuse. That being the case, it might not be as good as the shield and horn, but it would certainly be worth the cost to me.