With Windows 10 v1709 (Fall Creators Update), make sure you running the latest Crimson Relive 17.1x.x drivers with the 'compute' toggle on for each of the cards in AMD settings or the AMD blockchain drivers in Windows v1703. I have the same cards with the same dual Bios and get 31+ MH/s dual mining with DCR 875 MH/s with a core clock 1200 MHz memory clock 2100 MHz. I use the Polaris Bios Editor v1.6.2 'one click timing patch'. It automatically detects each memory type and adjusts the 1750 MHz and up straps with the bundled performance straps. PBE v1.6.3 and up adjusts the straps from 1500 MHz and up, but the 1750 MHz straps are more stable.

https://github.com/jaschaknack/PolarisBiosEditor/tree/9ec64066eecdb55ac86da7bc82181eaab2161d51

https://support.amd.com/en-us/kb-articles/Pages/Radeon-Software-Crimson-ReLive-Edition-Beta-for-Blockchain-Compute-Release-Notes.aspx