Depends on the tool.
Let's say one person can move 40 lb weight 100 feet before taking a break. Two people can move that same 40 lb weight 200 feet no problem.
Now let's say you have an 80 lb weight. Let's say one person can move it 40 ft. But it's hot out and he tires faster, so he only moves it 30 feet. Two people can move that 80 lb weight double the theoretical distance, to 80 ft, and they're less susceptible to the same heat-fatigue and get more than 2X actual distance.
Things are more complicated these days.
Same number of cells and same size cells? The lower capacity might be better for higher-drain applications. For lower drain, double the capacity gives you double the runtime.
For a 5-cell battery and 10-cell battery with double the capacity, you should get at least double the runtime. Some tools benefit from extra power, such as Milwaukee M18 drills and drivers.
But then you now have different sized cells, where a 5-cell 4.0Ah larger-cell battery is supposed to be comparable to a 10-cell 4.0Ah smaller-cell battery in terms of runtime and power delivery.
