Can you cast magic stone on 3 pebbles as a bonus action, throw a stone as a main action. Then ON YOUR NEXT TURN, throw the 2 remaining pebbles? Like 1st thrown as “main action” & 2nd thrown as your “bonus action”, inside the “two weapon fighting rules”, from PHB pg 195 below
When you take the Attack action and attack with a light melee weapon that
you’re holding in one hand, you can use a bonus action to attack with a different light melee weapon that you’re holding in the other hand. You don’t add your ability modifier to the damage of the bonus attack, unless that modifier is negative. If either weapon has the thrown property, you can throw the weapon, instead of making a melee attack with it.
Seeing as there is no, individual, entry for “pebble” and/or “stone” (making it an improvised melee weapon?), but also seeing as the attacker is considered “proficient” (it doesn’t state your at any disadvantage with the stones) with the magic stone & its 100% considered “thrown” (or used as sling ammo), could it ALSO be assumed to be “light” (like a dagger, light hammer or hand ax is?).
Seeing the limited use of bonus action’s spells when attacking ranged as a druid
Magic stone, Shillelagh & Healing word, as options. None are “direct attacks”.
Especially before level 5, when produce flame & create bonfire out pace magic stone as ranged attack options, being able to “dual wield” 2 magic stones would be a doubling of action economy & damage output at range, mainly for druids levels 1 to 4
Side note I have melee handled with dual wielding a shillelagh’ed club with an off hand scimitar, its pre-level 5 ranged options I hope to expand