It would work like this: Once a skill that can be combo'd with another is learned, you can right click the *First* skill in your hot-bar and it will bring up the normal context menu, but above the normal "Remove" option, there would be the option to "Create Combo Button" (or whatever it ends up being called, I'm not great at naming things). Then, a sub-hotbar would appear above the selected skill with the first skill and a box with a plus in it, to which you'd drag and drop the next skill on top of the second box, adding it to the combo.

The reason this should be implemented is it would allow skill-heavy classes to clean up their screen a bit and give them one less thing to worry about.