Can both injection units be run from one computer?

Yes ... depending ... you can go two routes.

Use the complete Nissan system (throttle body, computer, harness and sensors, including the oxygen sensor so it can self-tune). Use the stock FI setup for this. I can't recall if any use two throttle plates. Pick a system with a mass-airflow sensor for best results on different size engines.

Or, buy an aftermarket computer which offers more choices (simpler or more complex, take your pick). This is far more expensive.

If you're gonna make your own manifold, consider going with port injection (4 injectors, one in each intake runners). This avoids mixture distrubution problems.