i understand your plan and i know you could probably make it work BUT i disagree with your complicated approach.

my thoughts are if you have VSS, TPS, RPM, and MAP, and disable a lot of codes, you can likely get the 7427 to act as a standalone.

connecting the same sensor to two different ECMs can be problematic. they each send their own reference voltage. it can be a wiring nightmare although anything is possible if you understand analog signalling.

so here's a really big question for you

why not just convert the 7427 to batch fire mpi and dump the other ecm, having one ecm controlling everything?

were you aware the 7427 is plenty capable of feeding a TPI rig? it has been done quite a few times