You've said that the bosses want the networks to be separate.
You've also said that company B provides a service for company A akin to B providing email services to A, so A needs access to B's network for that single service.
All that said - what is the goal in splitting the networks? Why do it? If you don't know why the bosses want this - ask them. Let's not worry about the how of splitting yet allowing things to continue to work, let's work on the why first - because the helps lead to the correct solution for the goal.