Afaik it depends on the situation. If it's an efficient route and a driver accepts multiple orders from the same restaurant they might be able to take more than one. But in general they don't restaurant hop, and back in the day when I had friends who did Uber eats, it would have to be a choice by the person to pick up more than one order at a time, so it's one driver per restaurant per time window of accepting and order and delivering it at least. And that's probably only assuming you're in a busy city and not a slower rural area.

I only say it's unsustainable because after the food you order from the restaurant is made there are now two people in the chain for a meal for one person. That doesn't square up even before you factor in the cost.