Hi there. You can write your own classes like font awesome to make this work. The key to making this work is that in font awesome, the fa-stack class has a position relative, and the other two has position absolute.
So using the above html, just add this to the css.
.fa-stack { position: relative; }
.fa-stack-1x, .fa-stack-2x {position: absolute; top: 0; left: 0; }
And if they don't layer properly, then write individual classes for .fa-stack-1x and 2x and move the position inside it.