> You can also design software A to produce both G1 and G2 side by side

This can be impractical if G is a database table that is very large.

> it’s best imho to make the projects truly independent, as if they were developed by different companies.

One of our systems might cost £300k, so completely desynchronising them so that code paths can build both G1 and G2 simultaneously (allowing B to develop separately) means "simply" doubling the costs. That might put our team at a disadvantage against someone who figures out another way.