[Stu_D]

Your problem and it seem's to be similar to mine but all I wanted to solve
was using one battery only for the twin ignition system . This also follows
having only one ign cut , one manul switch and one SF opto cut switch .

All I did was to Y lead the line from the opto to the ignition power in .

I'll see if I have some photo's . One thing is for sure , it works well .
After doing a two hour groung using a 2400 A123 for the ing power
it didnt miss a beat . It used 1600mha over the entire period .

Its a bit hard to see but the lead with the cover @ top left of pic
is the twin end of the "Y" , both are power and both are coming
from one opto and one battery .

You may have to re join the Y to get the right in/out as I did
but after that was sorted it worket a treat .