How can I put two menus in the same div?

Try setting 'container' => false, in the wp_nav_menu() options array and using your own HTML to wrap the output.

<div class="outer-container-whatever-bootstrap-classes">
    <nav class="navbar-left other-classes">
        <?php 
            wp_nav_menu( array( [your_options with 'container' => false,] ) )' 
        ?>  
    </nav>
    <nav class="navbar-right other-classes">
        <?php 
            wp_nav_menu( array( [your_options with 'container' => false,] ) )' 
        ?>  
    </nav>
</div>