I'll answer this, as I have experienced this feeling myself. Any spell or item that requires BOTH hands will not alter what ever your left and right hand slots were beforehand. Also, that conjuration spell is known as "Summon Arniel's Shade", which requires, well, both hands to cast. Examples of other things that require both hands to cast: Greatsword/axes/hammers, bows, and most master level spells, like the thrall spells, dragon flesh, and the other big spells.
I'll answer this, as I have experienced this feeling myself. Any spell or item that requires BOTH hands will not alter what ever your left and right hand slots were beforehand. Also, that conjuration spell is known as "Summon Arniel's Shade", which requires, well, both hands to cast. Examples of other things that require both hands to cast: Greatsword/axes/hammers, bows, and most master level spells, like the thrall spells, dragon flesh, and the other big spells.