I had this setup:
CPU – AMD FX4300
GPU – NVIDIA 1060 6GB Mini by Zotac with May 2017 drivers
Powersupply – 650W
Today, I added NVIDIA 1060 3GB Mini by Inno3D
I was getting 300+ Sol/s with Nvidia 1060 6GB and was expecting around 600 Sol/s with additional Nvidia 1060 3GB added but getting 515 Sol/s
I found in MSI afterburner that the first GPU is now running only at 1500MHz clock speed and 57% power while the second GPU (newer one) is running at 1800MHz and 72% power. I’ve attached screenshots of afterburner of both 1060 6GB and 1060 3GB. Please help.
Is it due to lack of power?
Some improvement in Sol/s after swapping places. Now my 1060 3GB is stuck at 1506MHz. 
I am blaming my 650W PSU. 
Problem solved. Upgraded my PSU to 750W for extra $7 bucks and now getting combined 600+ Sol/s!