welcome

you can definitely get 'full manual' control over your 4l60e. i've seen it done. i just wish i knew how; i can help you -a lot- with your lt1, but i know virtually nothing about automatic trans tuning.

as far as your dual tbi setup, you might just look at getting an EBL flash unit which is already rigged up for 4 injector tbi setups, and is probably a far superior ecm than what you're already usin'. other than that, a homemade external injector driver (basically to handle the extra load) would do the trick too, or hack up your ecm. you could poach a dead tbi ecm for its injector drivers and just run them in parallel on a little homemade daughterboard or something. guess it depends how much soldering and experimentation you want.

but there's other factors, what about idle air control?... making a pipe that just links the idle control of both bodies to one iac motor would work