I'm pretty sure your Suzuki project bike uses a wasted spark ignition system which means it fires each cylinder's spark plug every time the cylinder approaches TDC (whether it's on the compression stroke or the exhaust stroke of the four-stroke engine cycle doesn't matter)One thing I observed is that when I have both inputs connected it gives correct RPM, when I have only one input connected I have half of the actual rpm. The b4 has an I4 motor with cylinders 1 & 4 firing off one coil and 2 & 3 on the other. Someone would expect that connecting to one coil would get the actual rpm number since there would be one firing event every 360 deg. Or am totally wrong?
So every time 1&4 approach TDC it fires that coil, and every time 2&3 approach TDC it fires that coil. With the Tach Driver Module hooked up to both coils it would be sensing an ignition event every 180 degrees of crankshaft rotation.
The only answer to the question has to be that this Tach Driver Module you're using (the one they designed for I4s and I2s that use two coils) is designed to divide the number of ignition events it senses by 2. So if you only hook up one coil and then the Tach Driver Module divides by 2, you'd get 1/2 the actual RPM.
(Are you actually living in Athens, Greece as your profile shows? News over here makes Greece sound like its just short of anarchy.)