multiple awaits vs Task.WaitAll - equivalent?

Short answer: No.

Task.WaitAll is blocking, await returns the task as soon as it is encountered and registers the remaining part of the function and continuation.

The "bulk" waiting method you were looking for is Task.WhenAll that actually creates a new Task that finishes when all tasks that were handed to the function are done.

Like so: await Task.WhenAll({getAllWidgetsTask, getAllFoosTask});

That is for the blocking matter.

Also your first function does not execute both functions parallel. To get this working with await you'd have to write something like this:

var widgetsTask = _widgetService.GetAllWidgets();
var foosTask = _fooService.GetAllWidgets();
customer.Widgets = await widgetsTask;
customer.Foos = await foosTask;

This will make the first example to act very similar to the Task.WhenAll method.

The first option will not execute the two operations concurrently. It will execute the first and await its completion, and only then the second.

The second option will execute both concurrently but will wait for them synchronously (i.e. while blocking a thread).

You shouldn't use both options since the first completes slower than the second and the second blocks a thread without need.

You should wait for both operations asynchronously with Task.WhenAll:

public async Task<IHttpActionResult> MethodB()
{
    var customer = new Customer();

    var getAllWidgetsTask = _widgetService.GetAllWidgets();
    var getAllFoosTask = _fooService.GetAllFos();

    await Task.WhenAll(getAllWidgetsTask, getAllFoosTask);

    customer.Widgets = await getAllWidgetsTask;
    customer.Foos = await getAllFoosTask;

    return Ok(customer);
}

Note that after Task.WhenAll completed both tasks already completed so awaiting them completes immediately.