No...you have 8GB VRAM total, but the way Oxide is doing things in Ashes, it will behave like four 2GB cards. Other games could choose to do things differently, but there's a massive bottleneck in pulling data from one GPU to another over the PCIe bus. At best, a 16-lane Gen3 PCIe bus would give you around 16GB/s of bandwidth. Compare that with the 384GB/s of local GDDR5 bandwidth on a GTX 690, and it's not even close.
So you can send frames or parts of frames over the PCIe bus without much difficulty (an entire 3840x2160 frame is about 33MB of data, so even at 60 FPS you're "only" using 2GB/s), but trying to access one GPU's VRAM via a second GPU is no better than trying to access data in system RAM from a CPU. An X99 rig with DDR4-2666 for example has a theoretical 85GB/s of system memory bandwidth. Not that you'd ever be able to use all that, of course, as you'd still be accessing it over the PCIe bus.