How do you mate two engines to one gear box?

I was on BAT and some VERY adventerous person decided to scale a 1:18 replica into a full on 917 with opposed Subaru Tribeca engines to make the boxer 12. question is, if the cracks are facing each other, how do they put power to the transmission and wheels? Is there a "gearbox" (I know that isn't right but I can't think of the technical term) that makes two right angles and then the shaft runs to the gear box? Also, how do you use a Tribeca auto transmission as a rear transaxel? I'm so confused by this home build.