this looks like a good read
(18 articles on a Hayabusa-powered Miata - including the adaptor to couple the Hayabusa engine to the Miata transmission)
http://www.motoiq.com/MagazineArticles/ ... abusa.aspx
similarly couple 2 Haya engines in V-ish/U-ish disposition via the (original clutch) gear on to a car transmission or transaxle ?
engine 2 lower case being replaced by a large adaptor plate attaching to the top of engine 1 lower case in the transmission region
precision positioned by design (for gear mesh) via the new shaft's bearing outer races
the adaptor plate may have its faces that mate to the engines at an angular offset to optimise engine positioning
if the V angle is small the engine counterbalancer shafts might be removed and the engines coupled about 90 deg out of phase
also this (magazine's) adaption could simply allow engine coupling (incorporating both engine's gear casings) as a straight 8 emulator
this could be transverse (with central takeoff to a transverse transaxle) or longitudinal
this adaption is roughly what I had in mind decades ago ....
for aircooled (twin cyl) motorcycle engine/reduction gear for homebuilt aircraft - the reduction ratio is naturally suitable
(yes, it'd need a high end thrust capacity bearing - but it's totally doable)
car units were and are less than suitable for this