Also I may have not give you the best techniques for what you need to do, because I don’t know about the finality of it. You didn’t tell me why you need two images stacked.
Depending on what you’re trying to do, we could also have used one element with two backgrounds, a combination of image + background, a combination of elements with backgrounds, a combination of elements containing images… lots of things are possible.
That’s a general advice for when you want to get help from here: show you goal first, before asking for specific. HTML/CSS offer a very large variety of solutions for every design question. And as Webflow designer is respecting HTML/CSS a lot… we have all those potential solutions at hand.