Great questions... but I am sorry to report I believe It is not as straightforward as was the case for thefour lane APB (C7042)...
I am working on a microcontroller interface which reads eight sensors across four lanes and sends the correct signals to the ARC PRO sensor input channels... of which there are four.
Design considerations include:
1/ sensors cannot be simply connected in parallel as was the case for APB. This is because the ARC PRO sensors include a blade detect feature which would in this case permanently block all signals.
2/ the pit exit sensors are integral to the lap measurement algorithm for the Scalextric ARC app... with the app needing to know the sequence by which cars pass the S/F sensors for a given lane. Running two lanes through the same sensor channel is likely to confuse this sensor reading algorithm. Here interestingly the Magic app uses a different method/algorithm which uses a timer delay instead of the exit sensors to avoid double counting of laps. So technically the Magic app is easier to adapt for 4 lane applications than the Scalextric app.
3/ the simple APB approach can of course miss laps if two cars pass the S/F line neck-to-neck. This can be corrected if careful sequencing of the sensor pulse streams is implemented.
4/ many users also want a pit lane which straddles the S/F lane too.
The design I am working on uses a microcontroller interface to address all four above considerations... in particular I am keen to provide a solution which will work with both Scalextric ARC and Magic apps.
While I am working on a robust full system solution... there is a potential short cut... a few logic gates as interface between the sensors and the powerbase and then... use the Magic app only.
I hope this helps....