We've tried the free version of NGUI and it's efficient, clean, easy to use and exactly what we're looking for.
90% of the time two buttons at once isn't a problem. It's mostly an issue with changing panels or displaying popups. For example: If you push a button that would display a popup at the same time as you push a panel change button, the popup will then display over the next panel, instead of the current panel.
There's definitely ways to structure button and panel change logic to avoid the issue, but we were just wondering if there was an easy way using NGUI to make it so that only one of a certain set of buttons could activate at a time.