I have a system of 2 1070s, one Gigabyte Windforce, the other Asus Aorus.
The problem is, dstm miner crashes when I use both cards.
I've similar problem when i tried same config one card directly in 16x slot and second in raiser. In my case second card didn't work at all. As soon as I put both cards in raisers it has started to work without problems.
I think when you connect one card directly to x16 slot on older mobos it use all PCIe lines and then other cards will not work properly.
For me, the problem seems to be somehow connected to temperature. Though the working temps are below the factory recommended
limit of 83 C, somewhere around 78 C. Right now I launched XMR mining on the second GPU, which is not so aggressive, both temps are 60-65, by now it is stable. However, the system used to operate mining Zcash at 75-78 C for weeks....