Using Bootstrap, you have 12 "units" in the page width. This means that if you have 2 columns of md-6 (which is half the screen width on a display of 992px+), you will never have a gap between them.
If you need the whitespace on a medium+ display, introduce a column between them and change their widths, like so:
<div class="col-md-5"></div><!--
<div class="col-md-2"></div><!--
<div class="col-md-5"></div><!--
With this we still have our 12 column units, but with a responsive gap between.
For more information about the Bootstrap grid system, get it from the horses mouth at:
CSS · Bootstrap[
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