Could I caste Flame Arrow on a some bolts and then Flame Arrow with Elemental Spell (Acid) applied (as a 3rd level spell) to the same bolts for an extra d6 of fire AND an extra d6 of acid damage?
As an aside, would the gold dragon sorceror bloodline (which adds +1 per die to fire damage spells) apply to the d6 of fire damage from Flame Arrow?
Flame Arrow
School transmutation [fire]; Level sorcerer/wizard 3
Casting Time 1 standard action
Components V, S, M (a drop of oil and a small piece of flint)
Range close (25 ft. + 5 ft./2 levels)
Target fifty projectiles, all of which must be together at the time of casting
Duration 10 min./level
Saving Throw none; Spell Resistance noThis spell allows you to turn ammunition (such as arrows, crossbow bolts, shuriken, and sling stones) into fiery projectiles. Each piece of ammunition deals an extra 1d6 points of fire damage to any target it hits. A flaming projectile can easily ignite a flammable object or structure, but it won't ignite a creature it strikes.
I'd say yes to both. I don't know if you're going to find an official rule, though someone might tell you that multiple castings of the same spell do not stack. If you want to burn two level 3 spells to do this, I'd let you though. And the plus 1 damage from Sorc is inconsequential enough that I'd not worry about it enough to try and figure out if it applied by rule.
I'd be inclined to say since they are both the same spell you couldn't do it for both fire and acid. However I would really have no problem letting it fly in a game I was running. Same with flame arrow on arrows shot from a flaming bow, 2d6 fire.
As for the dragon/fire bloodline yeah I think that would work, and it's something I never really thought of. You're only eking out an extra 1 damage per arrow, but hey :).