Its doable if you think mobile first. Place the divs in the order you want them to appear in small viewports and then reorder them for larger viewports.
<div class="row">
<div class="col-sm-12 col-md-6 col-md-push-6">
<div style="background:red">
Put the one that for small screen on top
</div>
</div>
<div class="col-sm-12 col-md-6 col-md-pull-6">
<div style="background:green">
Put the one that for small screen on bottom
</div>
</div>
</div>
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