Same class, same style-sheet, different font-family.

sample.less

@glyphiconFont: 'Glyphicons Halflings';
@fontAwesoneFont: 'FontAwesome';

// Glyphicons
@chevron-left: "\e079";
@chevron-right: "\e080";
@plus-sign: "\e081";
@ok-sign: "\e084";
@ok: "\e013";
@triangle-bottom: "\e252";
@menu-right: "\e258";
@icon-close: "\e088";
@map-marker: "\e062";

// Font Awesome
@fa-var-500px: "\f26e";
@fa-var-adjust: "\f042";
@fa-var-adn: "\f170";
@fa-var-align-center: "\f037";
@fa-var-align-justify: "\f039";
@fa-var-align-left: "\f036";
@fa-var-align-right: "\f038";
@fa-var-amazon: "\f270";

// Use to extend Glyphicon font-family
.gl {
	font-family: @glyphiconFont;
}

// Use to extend Font Awesome font-family
.fa {
	font-family: @fontAwesoneFont;
}

// Use only the icons you want, not the the entire list of variables
@icons1:
  chevron-right,
  ok,
  triangle-bottom;

@icons2:
  fa-var-500px,
  fa-var-adjust,
  fa-var-adn;

// Set 1
.icon {
    .-(@i: length(@icons1)) when (@i > 0) {
        @name: extract(@icons1, @i);
        &-before.@{name}:before,
      	&-after.@{name}:after {
          &:extend(.gl);
          content: @@name
        }
        .-((@i - 1));
    } .-;
}

// Set 2
.icon {
    .-(@i: length(@icons2)) when (@i > 0) {
        @name: extract(@icons2, @i);
        &-before.@{name}:before,
      	&-after.@{name}:after {
          &:extend(.fa);
          content: @@name
        }
        .-((@i - 1));
    } .-;
}