That was sure hard to follow. I think you have the concept, but it may help you to realize that the terminal labels on the board are just that: terminal labels on the board. By default, you would plug P1Button1 into the P1SW1 terminal and it will send LCtrl, but you could plug it into P3SW5, reprogram the board so P1SW5 sends L Ctrl, and that works as well. I'll explain why later.
I was just having a look a the key assignment cinfiguration table in WinIPAC tough and noticd that there seems to be an option to set a second shift key.
one for P1 & 2 and one for P3 & P4.
Am I right in assuming that you can set two different shift keys but they will only work with half the controlls?
EG like the pic, i could set shift 1 to be P1 start and shift two to P3 start.
That is my understanding.
but then shift 1 (P1 start) could only be used to shift keys plugged into P1 or P2 controlls?
same for the second shift.
To only shift switches connected to the P1SW1-8, P2SW1-8, Start 2, Coin 1, and Coin 2 terminals - subtle difference which I am getting to.
So I could theoreticaly have P1 coin set as shift which could control P2 P3 and P4 coin and start buttons by setting both shifts to P1 coin.
I didn't quite follow here.
You can't set both shifts to the same input. (In theory, I think you could set Coin 1 and Coin3 as shift inputs, wire a button to BOTH terminals, and re-program the coin3 terminal to send 5, but you would lose one input, would probably need diodes on the switch, and it still might not work. I know it is not recommended.)
Do you mean P1 Coin, Coin1 (5) is a shift button and when the Coin 1 button is pressed along with another key you get the following (using only one shift button):
P1B1 it sends P1 Start
P1B2 sends P2 Start
P1B3 sends P3 Start
P1B4 sends P4 Start
P1B5 sends P2 Coin
P1B6 sends P3 Coin
P1B7 sends P4 Coin
If so, yes you can do that, but it will be awkward having player 4 jump over to Player 1's position to play the games.
You can also set Coin 1 as the shift button and have:
P1B2 it sends P1 Start
P2B2 sends P2 Start
P3B2 sends P3 Start
P4B2 sends P4 Start
P2B1 sends P2 Coin
P3B1 sends P3 Coin
P4B1 sends P4 Coin
To do this, program it as above but do some tricks with the wiring.
For example, we want P3B2 to send P3 Start "3", but it is on the wrong side of the board. However, we wire it to P1SW8, program P1SW8 to send the normal P3B1 key (RCtrl?) and program it to send 3 when shifted. You will end up using up most of the P1 inputs, so the normal P1 buttons will need to be wired to P3SWx or P3SWx input (P3SW1, P3SW2, etc.) and these reprogrammed, but it will work.
But I couldn't set P1 coin as the shift and then have it shifting P2, P3 & P4 coin while also having P1 start as the second shift, shifting P2, P3, P4 start.
That is actually easier. You just wire your P1 Start button to the Start3 terminal, set Start3 as your second shift key, and reprogram Start 3 to send "1".
I probably lost you, but reply back and I'll try to do better.