Your meal, your leftovers.
If dishes were shared, whoever picks up the bill wins the leftovers.
If the bill is split, split the leftovers.
If the leftovers will not split easily, award them to the paying party that ate the least.
If both parties paid and ate the same amount, volunteer to surrender the doggie bag under the condition that you will irrevocably win the leftovers next time.
Really important, I know. Far too many people skirt the issue, and I wanted to set things straight.