I'm wiring these into my Z and looking for a few pointers?

 

Can the wires shared between the two be spliced into the point in the ECU harness? (Such as RPM, throttle) or are you better to have space between them?

 

And for the ground are you better using two separate ground wires from the ECU for each, or have all 4 onto a single ground? And if using a single ground which order should the grounds go in?

I.e AVCR1---SAFC1---ACR2---SAFC2

OR AVCR1--- AVCR2--- SAFC---1 SAFC---2 etc.

Can't help with your query, but the SAFC2 should be used for monitoring purposes only. Do not use it to alter fuelling, it fudges the signal from the MAF. The signal from the MAF determines not just the fuelling, but also the subsequent timing.