Place multiple bars into the same .progress to stack them.
<div class="progress">
<div class="progress-bar progress-bar-success" style="width: 35%">
<span class="sr-only">35% Complete (success)</span>
</div>
<div class="progress-bar progress-bar-warning" style="width: 20%">
<span class="sr-only">20% Complete (warning)</span>
</div>
<div class="progress-bar progress-bar-danger" style="width: 10%">
<span class="sr-only">10% Complete (danger)</span>
</div>
</div>