How early is too early to be involved in new feature development?

Question

I develop an internal business application that our engineers use to test and debug the company's product. The engineering manager has been pushing to get the software developers involved when the engineers are in their design phase. He says that he wants to give us an idea of what's coming down the pipe, and give us the opportunity to provide feedback on their designs (we rarely have feedback).

The engineering manager wants the developers and engineers to implement new features simultaneously, so that by the time the engineers have implemented their end, they can use the software to test them. The feature designs tend to be volatile until they are fully implemented by the engineers, making it difficult to draw up requirements. He thinks the engineers and developers can do all of our debugging side-by-side, at integration time.

It all seems inefficient and backwards to me.