Given that the guidelines for CrHe spell out quite specifically that finished and treated plant products can be achieved with a base 3 start, I would probably design the spell thusly:
CrHe(An, Te, Re)
These arrows look like two strands of wood woven around each other in a spiral. Upon firing they burst into a cloud of 1000 arrows with feathers for fletching and sharp flint arrow heads. These arrows fly in roughly the same direction as the initial arrow but tend to spread out (as a group) making them most effective against large groups.
base 5 (already includes the An & Te requisites in the base) i.e base 3 +1 An, +1 Te (yes this would allow proper iron tips),
+2 Voice (perhaps even +3 sight, but Voice range minimum),
+2 group, +1 Rego effect, +2 size)
I would increase the range to Voice to account for range to target since the effect is not activated by touch nor can it achieve group damage once it reaches touch range to target.
The effect must be triggered whilst in flight and I would argue that the activation would needs be a shouted vocal command of some sort. Sight might be best to allow for maximum non-magical range to target but you are seeking economy so Voice is most reasonable compromise.
What you have then is at minimum a level 40 effect. Given that base individual for Herbam is a plant roughly one pace in each direction (and by example of other CrHe plant spells, roughly equivalent to 10 paces of plant matter) then I would calculate that each arrow could give rise to roughly 20 duplicates (i adhere to the true meaning of pace @1.46m) before the size modifier is brought in.
(a medieval arrow was roughly half the height of the arher or ~75 cm. 14.6m/.75 yields close enough to 20 arrow lengths of material)
+2 size modifier would be a factor 100 increase from that and thus approximately 2000 arrows from a single arrow. In which case you may not need the whole entwined dual arrow format at all (which wouldn't fly very well to be honest).
At level 40 with a single enchanted arrow you could have enough coverage to possibly route an entire mundane battalion. get off a few and you've likely routed if not slaughtered an entire brigade! lol.
Thats my mythic pound's worth anyway!