Set CSS class for several ID

I have done the following:

#my-el .some-class {
    width: 10%;
}

That works but now I'd like to apply the same class to the element #my-new-el. I've tried:

#my-el #my-new-el .some-class {
    width: 10%;
}

But this does not work. What would be the correct syntax to achieve this ?

via Chebli Mohamed