I was able to use two getImageList calls, one in each column, each with a different conditional based on [[+_alt]]:
[[+_alt:is=`1`:then=`
<div class="col-md-12 col-sm-12 col-xs-12 cl-stacked-margin-bottom">
<div id="cl-tesimonial">
<div class="testimonial-content"><i class="fa fa-quote-left" aria-hidden="true"></i>
[[+client_testimony]]
<h2 class="cl-client-name">[[+client_name]]</h2>
<div class="cl-author image-testimonial"> <img src="[[+client_img:notempty=`[[+client_img]]`:else=`[[++assets_url]]img/client_images/testimonial.jpg" alt="[[+client_name]]">
</div>
</div>
</div>
</div>
`:else=``]]
and:
[[+_alt:is=`1`:then=``:else=`
<div class="col-md-12 col-sm-12 col-xs-12 cl-stacked-margin-bottom">
<div id="cl-tesimonial">
<div class="testimonial-content"><i class="fa fa-quote-left" aria-hidden="true"></i>
[[+client_testimony]]
<h2 class="cl-client-name">[[+client_name]]</h2>
<div class="cl-author image-testimonial"> <img src="[[+client_img:notempty=`[[+client_img]]`:else=`[[++assets_url]]img/client_images/testimonial.jpg" alt="[[+client_name]]">
</div>
</div>
</div>
</div>
`]]