Bootstrap 4 Multiple fixed-top navbars

Yes, it’s possible but you have to position the 2nd one accordingly. The height of the Navbar is ~56px.

.fixed-top-2 {
    margin-top: 56px;
}

body {
    padding-top: 105px;
}

<nav class="navbar navbar-toggleable-sm bg-faded navbar-light fixed-top fixed-top-2">
    <button class="navbar-toggler navbar-toggler-right" type="button" data-toggle="collapse" data-target="#navbar1">
        <span class="navbar-toggler-icon"></span>
    </button>
    <a href="https://stackoverflow.com/" class="navbar-brand">One</a>
    <div class="navbar-collapse collapse" id="navbar1">
        <ul class="navbar-nav">
            ..
        </ul>
    </div>
</nav>

<nav class="navbar navbar-toggleable-sm bg-inverse navbar-inverse fixed-top">
    <button class="navbar-toggler navbar-toggler-right" type="button" data-toggle="collapse" data-target="#navbar2">
        <span class="navbar-toggler-icon"></span>
    </button>
    <a href="https://stackoverflow.com/" class="navbar-brand">Two</a>
    <div class="navbar-collapse collapse" id="navbar2">
        <ul class="navbar-nav">
            ..
        </ul>
    </div>
</nav>

Demo: Bootstrap 4 Multiple fixed-top Navbars

In some scenarios, it may be better to use a single fixed-top DIV to contain both.

Also see:
Bootstrap 4 Navbar sticky after header
Bootstrap 4 collapsing two navbars into one toggle button