I have several users who need two line extensions, such as Support which has a second extension on a ring group for incoming support calls (they are not interested in queues yet). This causes multiple buttons to show up in FOP2, and sometimes they are not listed next to each other, since FOP2 sorts by extension number (maybe this could just be handled by providing a sorting option--sort group by name or extension). But you could also tie these multiple extensions to one button. It could be a global option, though I could see the odd-man-out requiring more granular control (one extension in a Support group and another extension in an Internal Calls group).
And if they were tied to one button, ideally there would be control over which extension is their primary preferred extension for incoming calls (ability to sort the order that they appear in the button).
Food for thought. Would also be a nice feature for receptionists, so they can find users faster and not have multiple entries for each.