Yes, spells are kinda weak damage wise compared to dual wand (or magus staff spell).
Dual wand progression is like that (compared to just one single wand at 0 point)
You put a wand in your offhand and you get
-30% damage per AP
+1 dualwield: -8%
+2: +20%
+3: +35%
+4: +50%
+5: +100%
So, indeed, +2 is when dualwield becomes on par with wand skill. But I don't know if you should stop there. I never saw duelwield bonuses elsewhere than belt and gloves, so I assume you could stop at 3 if you are happy to count on luck for those two slots.
