GPU Rendering? Theoretically the new HP Z6 and Dell T5820 both fits two Titans (dual slot) graphic card. But it’s not recommended and an ideal solution. Of course the Z8 and T7920 are better suited.
Keep in mind that the cooling of the GPUs is essential!! Otherwise you have to deal with throttling which means reduced performance.
IMHO my advice is: build your own workstation with proper cooling and power supply (GTX card need more watts then Quadro cards).