I assume that you are using the ASCOM driver to connect your cameras and not the native driver. With the ASCOM, there are two drivers: one for camera 1 and one from camera 2. You have to use the camera 1 driver for one camera say your ASI1600MM and the camera 2 driver for the guide camera say your ASI120MM. Or you can use the native for the giude camera and the ASCOM for the image camera. I use Sequence Generator Pro and use the ASCOM connections without issue. I have even used the two ASCOM drives for two imaging camera collecting at the same time and the native driver for the guide camera. What main imaging software are you using?
One issue with the ZWO cameras that I wish they would change is to have a third ASCOM driver for the guide scope so that I can not use the native driver. The native driver is on 8 bit and I get much better guiding with the ASCOM driver at 16 bit. QHY has such a third guide camera ascom driver. I have requested this from ZWO twice and they say its not worth it. If you can, ask for this on the ZWO forum.
I should be able to get this working for you with more details. For the native in PHD2, there is a button on the connection popup that allows you to select which camera you want to use.
Have you updated to the latest ZWO drivers on the web page.?