According to Scitsu's fitment list connecting the pickup wire to both HT leads is correct for the "black" version of their rev counters. So unless they don't know how a twin magneto works (unlikely), changing the way the sensing lead is attached prolly won't be to any avail. Still, worth a try.
FWIW, I have recently tried one of these as a temporary way of monitoring revs on the A10 (just out of interest):
http://www.ebay.com/itm/Tach-Hour-Meter-For-Motorcycle-ATV-Snowmobile-Boat-Gas-Engine-Tachometer-Bike-/280929962699?pt=LH_DefaultDomain_0&hash=item4168bb0acbThe most useless piece of c**p I have bought in a long time. It takes about 4 seconds to react to any change in revs at all, and won't go above 4800 rpm, flat out, despite all my best efforts in trying different ways of attaching the pickup wire(s), numbers and direction of coiling, changing leads, connecting and disconnecting the ground wire etc. etc. Not worth the money, however little it is.
The hour meter option works okay though, so maybe useful for an old tractor... (non-Diesel
)