Hi Jean,
There’s a couple options.
1. You can use the tagline as the second line – and style it to look the same using the Typography addon.
2. Hide the site title and tagline in the Customizer, then add the following to the “Before Header Content” hook in GP Hooks:
<div class="site-branding">
<p itemprop="headline" class="main-title"><a rel="home" title="Your Title" href="http://yourdomain.com">Your first line of the title <br /> Your second line of the title</a></p>
</div>
Let me know if that makes sense or not ๐