Whereas I happened upon previous yojis by happy circumstance or through random J-kaiwa, this one I actually acquired through blunt force (ie, asking a friend "What are your favorite yoji?"). In Japan, this can be seen as a kind of personality test. First, ask somebody what the first 四字熟語 that pops into their mind is. Anything is fair game. Next, ask them what their favorite yoji is, with the distinction that it should probably be different than the first (though it doesn't have to be. Their first reply is supposed to reveal their perspective on life, and their second is supposed to reveal their perspective on love.
I was first subjected to this two-step personality quiz by my girlfriend, who despaired in the answers my limited knowledge of such phrases yielded: 四面楚歌 and 中途半端. So in life I feel like I'm surrounded by enemies, and my affections are half-assed. It took some fancy footwork to convince her that the test was bogus outside the realm of native speakers (hell, I still think 凸凹 is the best kanji EVER), but if you don't want to suffer the same fate, you'd best keep comin` back.
Hmm - 凸凹 (でこぼこ) is actually a pretty good segue here...
1. Two sides of the same coin
2. Two parts of the same thing that are inexorably linked with one another.
3. You can't have one without the other.
I still haven't completely pinned down the usages of this one, but they are wide-reaching and can be applied to a lot of interesting situations. Computer hardware and software, for instance, came up a few times in my research. Just having the hardware or the software is meaningless - you have to have both. In this way, the yoji works for two things that define eachother's utility.
But beyond that, you can also apply it to situations where the two 'sides' are conflicting instead of complementary. For instance, being a professional football (AMERICAN, that is) player might seem like a glamorous proposition. But the other side of that coin is a ton of hard work, sometimes unwanted attention, and risking your body and career every game you play.
An even subtler usage is provided in an example from my 電子辞書:親切とおせっかいは表裏一体だ. Kindness and meddling are two sides of the same coin. Even if you are just trying to help someone out, that assistance can be misconstrued as condescension or downright interference. "The road to hell..." and all that.
My challenge for you all is to try and use this word in a creative way in daily conversation, and to post your results here. To make it harder, you can't use it in any of your many conversations concerning Newtonian Physics, because that would just be cheating.
Batman's enemy "Twoface" is the very embodiment of the phrase "two sides of the same coin." From his suit to his scarred face, he is a walking contradiction. He even has a scratched up coin, for Christ's sake!
Fun, 表裏一体 links!