demo.html

<!DOCTYPE html>
<html>
<head>
	<title></title>
</head>
<body>
<script type="text/javascript">
class ProxySandbox{
	constructor(){
		const rawWindow = window;
		const fakeWindow = {};
		const proxy = new Proxy(fakeWindow,{
			set(target,p,value){
				target[p] = value;
				return true
			},
			get(target,p){
				return target[p] || rawWindow[p];
			}
		})
		this.proxy = proxy;
	}
}

const sandbox1 = new ProxySandbox();
const sandbox2 = new ProxySandbox();
window.a = 1;
((window) => {
	window.a = 'hello';
	console.log(window.a)
 })(sandbox1.proxy);

((window) => {
	window.a = 'world';
	console.log(window.a)
 })(sandbox2.proxy)
</script>
</body>
</html>