I'd imagine you want something like this in Bootstrap with the .scrollable divs set to have "overflow:auto" on them.
Bootstrap will make then stack automagically at lower resolutions.
<div class="row-fluid">
<div class="span6">
<div class="scrollable"></div>
</div>
<div class="span6">
<div class="scrollable"></div>
</div>
</div>