Two speeduinos, one for each bank, separate map readings but all other sensors are shared. 

•  Do you use some sort of can bus conection to sync them or just by sharing the sensors its enough? 

•  it’s also interfacing with the oem controller which puts rpm on canbus for dash and fuel pump etc.  5v ref sensors simply share signals.
  Temp sensors share signals with bias resistors and conditioning capacitors removed. The bias resistor value is that of the factory controller.
  The key is to make sure the sensors and speedys are grounded the right way.

• Very dependant on engine layout though. This is effectively two straight six’s on a common crank. So taking separate MAP readings and running them individually is pretty good.

• the temp sensors only require one bias resistor regardless of how many ecus you have.
  
  In my case 3 ecus, the bias resistor is in the Mercedes controller so I removed both bias resistors on the speeduinos.
  
  The other sensors you just splice the signal line and feed it in on the corresponding pin.

• how to sharee rpm signal ?

• just splice and share the wires, make sure the grounds are common. No pull-up on speeduinos.

• Twinspark and no room. The 4 post coils are cheap and compact., same way a lot of big engines run. Just means you have to have tunerstudio open twice. One controller is the master and the other is just a slave for ignition and injection

• ​

 What kind of ignition is it? Distributor, or waste spark?

wasted spark. Example no good for HHo H2 opr GTNT  as we need constant + ground 

Each ign signal shared by two cylinders through a vag 4 post coil. So each ign signal is triggering 4 plugs to accommodate the twinspark system.

​I have an old M73 in storage, thinking of running the distributors, and only use one ignition output pr distributor.

 you use two ignition output pr ecu? And with 4post coil (16cylinders), you have 4 sparks not used?

• 
  Ign 1 to cyl 1&6 (4 plugs)
  Ign 2 to cyl 2&5
  Ign 3 to cyl 3&4
  
  Mirrored on the other bank