Modded up, but I really hope most frontend folk on HN already know about border-box. Short ver: float two 50% widths beside each other. Change padding as you like later and not have stuff break. I have been doing this for a year now, it's godlike especially for in-browser design.
Border box is also a godsend when dealing with nicely laid out form inputs because unlike elements like divs you can't get away with nesting child elements to get the desired effect, and you typically have borders on those fields. Need a textarea to fill a certain space? Set it to 100% and forget it.
From my understanding most browser use border-box on form elements by default anyway. That's why they often look different from other elements with the same styling.