You need version of Deus Ex: Mankind Divided v1.12 build 667.0 and win10 with all updates and last AMD driver
I'm not sure about last patch, but in earlier versions MGPU could be activated by this instruction:
To activate Multi-GPU DirectX 12 you are required to do the following:
- Within your Steam Library, right-click on Deus Ex: Mankind Divided.
- Select ‘properties’ from the context menu and navigate to the Beta tab.
- From the dropdown menu, select dx12_mgpu_preview then close the window.