Combine multiple bootstrap classes into one SASS class

Even though it is not a good practice, you should be able to do this via @extend. In your case, your code should look like this:

.label-width{
   @extend .col-xs-2
   @extend .col-md-3
}

Have a look here if you need more explanation.

you can extend multiple class using multiple way like:

Way 1

.my-class {
   @extend .class1;
   @extend .class2;
   .
   .
   .
   @extend .classn;
}

way 2

one another way is you can extend multiple class in an single line here example

.my-class {
   @extend #{'.class1', '.class2', ... , '.classn'}
}

Know more details: Sass official document

Thanks