回答

<script>
// AJAX call 1
function call_1() {
	return new Promise(function(resolve, reject) {
		let xhttp = new XMLHttpRequest();
		xhttp.onreadystatechange = function() {
			if(this.readyState == 4) {
				if(this.status == 200)
					resolve(this.responseText);
				else
					reject('Call 1 Failed');
			}
		};
		xhttp.open("GET", "ajax-1.php");
		xhttp.send();
	});
}

// AJAX call 2
function call_2() {
	return new Promise(function(resolve, reject) {
		let xhttp = new XMLHttpRequest();
		xhttp.onreadystatechange = function() {
			if(this.readyState == 4) {
				if(this.status == 200)
					resolve(this.responseText);
				else
					reject('Call 2 Failed');
			}
		};
		xhttp.open("GET", "ajax2-2.php");
		xhttp.send();
	});
}

Promise.all([ call_1(), call_2() ]).then(function(values) {
    // all AJAX requests are successfully finished
    // "values" is array containing AJAX responses of all requests
    console.log(values);
}).catch(function(reason) {
    // one of the AJAX calls failed
    console.log(reason);
});
</script>