When I leave multiple treasures behind at the end of the battle, the result is typically that it only gives me the contents of ONE of those bags, and all the others are completely lost.
You'll notice this a lot more later on when you get wide-range spells and can go back to large groups of weak enemies and clear the field in a few casts.
So if the enemy has any loot at all that you're interested in - or even if you just want to maximize your coins from a fight - I definitely don't recommend leaving more than one treasure behind at the end of the fight.