When you discover the identification of the android and are deciding to turn him in or warn him, go to the android first. Convince him to go to Zimmer on his own and he will reward you. After that, go to Zimmer in the science lab. Tell him about the android's identity. He will also reward you. Note: You will still get bad karma.

You can get both rewards if you go to Harkness (the android) first and agree to help him. One option when you agree to help him is gaining permission to kill Dr Zimmer and his bodyguard. You will get the plasma rifle from Harkness and then can go to the science lab. At that point you can betray Harkness and tell Zimmer his identity. You will then gain the VATS Perk from Zimmer. When Zimmer attempts to leave to look for Harkness, you can still kill him and his bodyguard with no penalty from the locals. When you return to Harkness he is none the wiser, and acknowledges that you have killed Zimmer. By doing this you will get both rewards and Harkness remains in Rivet City.