generally, you will want to use the best slow weapon you got in mainhand, the the best of "whatever" in offhand.
If you got, say, a ilvl 496 fist weapon, an ilvl 483 fist, and a 489 dagger, you will wont to go ilvl 496 fist / ilvl 489 dagger
if got a ilvl 489 fist though, and a 483 dagger, you will want to go fist/fist
with equal offhand weapons, you will generally want to use the slow weapon, or the one with the best secondary stats.
but in the end, with equal offhand dagger/fist, you will do more dammage with killing spree with the fist, but more poison dammage with the dagger. in the end they will do almost equal dammage, but slow/slow pulls ahead by a werry small amount.