The twoverdrive came out of the practical need of, while doing most of the overdrive parts with an Ibanez Supertube (the STL, I quit using my ST-9 live) or a standard TS-9, sometimes needing that extra power. My usual choice for the "Hard 'n Heavy" leads is either a RAT or, more often, an Ibanez Fatcat. Basically the same thing. Since the bars, pubs and other places where I play are usually too small rather than "plenty of space" all area that is not taken up with pedals is more than welcome.
For that reason I built the Twoverdrive, containing 2 drives and, as you may have guessed, it's a TS and a Rat. The TS comes first, the RAT second. That means that, if willing, the TS is able to boost the RAT.