I think you mean dualling a Wizard Slayer to a Mage. A Wizard Slayer is a much overlooked and maligned class, because of all the restrictions(you can't wear rings, cloaks, amulets etc.) See my below post at
http://www.gamebanshee.com/cgi-bin/u...c&f=2&t=007179 for a larger discussion.
The problem with a WS/Mage is not that you couldn't cast spells(not a problem), but that you couldn't use much equipment. Without even armour to lower your AC, you will probably not get below AC 0. If you have Throne Of Bhaal, then a WS/Thief is good for the high-level ability Use Any Item, which negates the restrictions. But in any case, you probably want to dual at lvl9 if you haven't got ToB, and 12 if you have.