I have tried over a half-dozen solutions suggested on Stack Overflow, and the only thing that worked for me was this:
<div class="row" style="display: flex; flex-wrap: wrap">
<div class="col-md-6">
Column A
</div>
<div class="col-md-6">
Column B
</div>
</div>
I got the solution from https://codepen.io/ondrejsvestka/pen/gWPpPo
Note that it seems to affect the column margins. I had to apply adjustments to those.