JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/3.3.4/js/bootstrap.min.js"></script>
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/3.3.4/css/bootstrap.min.css">
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/select2/4.0.0/css/select2.min.css">
<script src="https://cdnjs.cloudflare.com/ajax/libs/select2/4.0.0/js/select2.min.js"></script>
<nav class="navbar navbar-dark navbar-fixed-top">
    <div class="container-fluid">
        <div class="navbar-header">
            <button type="button" class="navbar-toggle collapsed" data-toggle="collapse" data-target="#navbar">	<i class="fa fa-chevron-down fa-2x" style="font-size: 16px;color:#fff"></i>

            </button>	<a class="navbar-brand">NAME</a>

        </div>
        <div class="collapse navbar-collapse" id="navbar">
            <form class="navbar-form navbar-left" role="search">
                <div class="form-group">
                    <select multiple="multiple" id="Search1"></select>
                </div>
            </form>
            <form class="navbar-form navbar-left" role="search">
                <div class="form-group">
                    <select multiple="multiple" id="Search2"></select>
                </div>
            </form>
        </div>
    </div>
</nav>

CSS

* {
    border-radius: 0 !important;
}
body {
}
#Search1, #Search2 {
    height:34px;
}

JavaScript

var data = [{
    id: 0,
    text: 'enhancement'
}, {
    id: 1,
    text: 'bug'
}, {
    id: 2,
    text: 'duplicate'
}, {
    id: 3,
    text: 'invalid'
}, {
    id: 4,
    text: 'wontfix'
}];

// Assume that each select will have as width the 1/3 of the body width
// Get body width and divide it with 3 and apply it to each select
var width = $('body').width() / 3;
$('#Search1, #Search2').width(width);

$("#Search1, #Search2").select2({
    data: data
});

// Put same code to the window.resize handler to run again when the window is resized
$(window).resize(function() {
    var width = $('body').width() / 3;
    $('#Search1, #Search2').width(width);

    $("#Search1, #Search2").select2({
        data: data
    });
});