Most of the time that secod pcie x16 is really x4 electrically so there is really no point of using that. If you want to use 2 gpus or other pcie demanding cards you should have x370. Second PCIe x16 is in x8 electrical mode.
Turns it out works or at least did for about a day maybe a little more. Last Sunday night the computer suddenly started to power itself off after mining for about 2 hours. Had been fine before that for a while. Most likely this is a motherboard or PSU that is failing. Pretty lame if it is the expensive seasonic titanium I put in this system. Not sure yet, busy trying to test the GPUs and make sure they are not the issue.