In every company I have worked for in the last 10 years, the layer in between business experts and software engineers has been always: the manager (either product manager or eng. manager). It's a bottleneck. The flow usually goes like this:
business expert -> manager -> engineers -> manager -> business expert -> manager -> engineers -> ...
Whenever the manager comes with "requests" from the business side, we end up with tons of questions because everything is half-baked. Manager goes back to business with our questions and comes back with some answers, but usually that just generates even more questions. In this way managers feel empowered. There's no way they can let the engineers just talk with business (otherwise the managers would feel like they are not contributing in anything... which is kinda right if the engineers are more or less professionals).