I believe to get what you want, a variable is best used:
$yourWidth: 100px;
.class1 { width: $yourWidth; }
.class2 { margin-right: ($yourWidth + 2); }
Update (based on comment info)
You might add a global variable below $ColCount that begins as an empty list, like so:
$WidthList: ();
Then inside @mixin columns($numCols) after $colWidth is calculated, add the width value for that column to the $WidthList by adding this function:
join($WidthList, $colWidth);
Then, once all the columns have calculated, you should have a list containing all the width values, so that you can access them if you desire elsewhere, and thus...
.class2 { margin-right: (nth($WidthList, 5) + 2); }
...should yield the .cl-col5 value you want for the margin.
Note that I did not test this. Nor have I actually ever used SASS. I am basing this strictly off the documentation found here and elsewhere on their site.