Should be possible, they're in the same family, one just has disabled shaders. However I would not advise any multiple GPU setup, especially when you're talking about spending over $1000 for just a single game, not even counting the cost of the monitor, especially in the age where multi GPU performance is solely in the hands of the developer with Vulkan (which DOTA 2 is). Plus going by this (slightly dated) chart, and knowing Vega 64 is a touch more than 50% faster than the R9 Fury, you shouldn't have any problem getting around 120 FPS at UHD, and that's where Freesync comes in.
No, this is not supported. If you want to MGPU you'll need to use the same GPU, ie 2x Vega 56 or 2x Vega 64.