Now that's a new one on me-a Retro twin...! I've vague memories of seeing a Retro single somewhere (Clanford's book?) The whole issue of twins that do...and twins that don't.....seems ripe for a decent investigation-but it would be a monumental effort to do it and require the coordination of and input from a wide number of owners, since I doubt anyone has all of them...other European ones that spring to mind are the previously mentioned HP 120, the Italian B&C 61-and there have been a number of 1-off or low production number Czech and German twins...
That they're still a challenge even to experienced modellers was brought home to me last weekend when one of our WMAC members who'd just re-engined his glider tug with a 60cc Chinese horizontal twin ignition engine (he showed it off at the last club meeting) reported no end of problems getting it to run on two cylinders-and this a brand new engine-eventually sorted with a new (and hotter) plug in the offending cylinder. Seems the more things change, the more they stay the same...

ChrisM
'ffkiwi'