JSFiddle - React, Tailwind, and code Playground

HTML

<form>
    <input type="email" name="email" id="useremail" placeholder="Email Address" required> <br>
    <input type="tel" name="phone" id="userphone" placeholder="Phone Number" maxlength="10" required> <br>
    <input type="submit" id="sidebarformsubmit" value="Submit">
</form>

JavaScript

//DOMcontentloaded?

var email = document.getElementById('useremail'),
    phone = document.getElementById('userphone');
function onchange(){
    email[phone.value?'removeAttribute':'setAttribute']('required','required');
    phone[email.value?'removeAttribute':'setAttribute']('required','required');
}
email.addEventListener('change',onchange);
phone.addEventListener('change',onchange);