This doesn't work:

'My\AddNumbers' => DI\object()
        ->method('addNumber', 1)
        ->method('addNumber', 2),

Only ->method('addNumber', 2) is taken into account.

The method ClassDefinitionHelper::method() (code here) uses the method name as a key for the array that holds methods that should be executed on the constructed object but if you try to call the same method twice it will obviously not work as the array key will be overridden, always retaining only the last method call.