You can use the flex-column flexbox utility class to stack the 2 navs vertically next to the icon. This sets flex-direction: column on the navbar-collapse div so that it’s child elements stack vertically.
<nav class="navbar navbar-toggleable-md navbar-inverse bg-inverse">
<div class="container">
<button class="navbar-toggler navbar-toggler-right align-self-center mt-3" type="button" data-toggle="collapse" data-target="#navbarCollapse">
<span class="navbar-toggler-icon"></span>
</button>
<h1 class="py-2 ml-lg-2 mx-3"><a href="#"><i class="fa fa-envelope-o fa-lg mt-2" aria-hidden="true"></i></a></h1>
<div class="collapse navbar-collapse flex-column ml-lg-0 ml-3" id="navbarCollapse">
<ul class="navbar-nav">
<li class="nav-item active">
<a class="nav-link" href="#">Home</a>
</li>
<li class="nav-item">
<a class="nav-link" href="#">Product</a>
</li>
<li class="nav-item">
<a class="nav-link" href="#">Shop</a>
</li>
<li class="nav-item">
<a class="nav-link" href="#">About</a>
</li>
<li class="nav-item">
<a class="nav-link" href="#">Events</a>
</li>
</ul>
<ul class="navbar-nav flex-row mb-2">
<li class="nav-item">
<a class="nav-link py-1 pr-3" href="#"><i class="fa fa-facebook"></i></a>
</li>
<li class="nav-item">
<a class="nav-link py-1 pr-3" href="#"><i class="fa fa-instagram"></i></a>
</li>
<li class="nav-item">
<a class="nav-link py-1 pr-3" href="#"><i class="fa fa-twitter"></i></a>
</li>
</ul>
</div>
</div>
</nav>
Demo – Bootstrap 4 navbar with 2 rows
Here’s another variation with the 2 rows and right align search form:
<nav class="navbar navbar-toggleable-md navbar-light bg-faded">
<button class="navbar-toggler navbar-toggler-right" type="button" data-toggle="collapse" data-target="#navbarCollapse">
<span class="navbar-toggler-icon"></span>
</button>
<a class="navbar-brand my-auto" href="#">Brand</a>
<div class="collapse navbar-collapse flex-md-column" id="navbarCollapse">
<ul class="navbar-nav ml-auto small">
<li class="nav-item active">
<a class="nav-link" href="#">Shop</a>
</li>
<li class="nav-item">
<a class="nav-link" href="#">Products</a>
</li>
<li class="nav-item">
<a class="nav-link" href="#">Team</a>
</li>
<li class="nav-item">
<a class="nav-link" href="#">About</a>
</li>
<li class="nav-item">
<a class="nav-link" href="#">Events</a>
</li>
</ul>
<form class="form-inline ml-auto">
...
</form>
</div>
</nav>
Demo – Navbar with 2 rows on right
Update for Bootstrap 4.0.0:
https://www.codeply.com/go/05hEHoiUvv
Here is an alternate scenario with 2 navbars with one mobile toggler
Bootstrap 4.0 – Two NavBars that collapse into one toggle
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How move ‘nav’ element under ‘navbar-brand’ in my Navbar
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