The same reason that if it takes you three hours to drive to the beach alone, having two more people in the car with you doesn't make you get there in one hour. GPUs simply don't work together in this way.
For a long time, there were SLI/Crossfire options that *did* allow the same GPUs to work together, but it never really scaled that well, and neither the GPU manufacturers or the software developers had a real motivation to devote resources towards a very small niche rather than something more useful. DX12 did implement the ability to combine different GPUs, but nobody really ever used that feature, and it didn't spread beyond Ashes of the Singularity.
Unless you're the world's biggest fan of Ashes of the Singularity, there's no gaming purpose to having the 1650 installed. There may be other reasons, such as dedicating one GPU to encode a stream or particular kinds of workstation uses and so on. But it won't add any FPS to your games.