Why don't two reCAPTCHA sent via ajax work?

Question

JavaScript

The bottom line is that there are two forms, you need to insert two invisible recaptchas into them and send these two forms via ajax.
HTML code:

<script type="text/javascript" src="/main.js"></script>
<script src="https://www.google.com/recaptcha/api.js?onload=onloadCallback&render=explicit" async defer></script>
<form id="ShoppingCartFormFast">
    <div id="recaptchaFast"></div>
    <input type="button" id="SubmitOrderingFast"  onClick="CheckBeforeSubmit('#ShoppingCartFormFast');" value="отправить" />
</form>

<form id="ShoppingCartFormStandart">
    <div id="recaptchaStandart"></div>
    <input type="button" id="SubmitOrderingStandart"  onClick="CheckBeforeSubmit('#ShoppingCartFormStandart');" value="отправить"  />
</form>

Main.js content:

var captchaFast, captchaStandart,
    onloadCallback = function() {
    mysitekey = 'blabla';
    captchaFast = grecaptcha.render('recaptchaFast', {
        'sitekey' : mysitekey,
        'size' : 'invisible'
    });

    captchaStandart = grecaptcha.render('recaptchaStandart', {
        'sitekey' : mysitekey,
        'size' : 'invisible'
    });
};
//Проверка формы
function CheckBeforeSubmit(form){  
        console.log(grecaptcha.Respnse(captchaFast));
}

However, grecaptcha.Respnse(captchaFast) always returns an empty value. Why ?