I only have experience with Crysis 3. But this worked for me...
Phenom II x6 1090t @ 4.0GHz
2 x Radeon HD 7950's @ 1000/1250 (* stock for temps/fan noise)
All details maxed
2x SMAA
1920 x 1080 resolution
35-60FPS (* VSync was @ 60 so higher FPS in some areas is likely)
Average FPS was 50+ 90% of the time
It stays pretty much pegged at 60FPS. The only time it drops a little, and briefly, is during gunfights with a lot of FX on screen in large areas with several AI enemies moving against my position.
There was no bottleneck I could see. Felt very balanced. The CPU and both GPUs would go from 50-90% usage on all CPU and both graphics cards, depending on the situation.
This was also on a GA-990FXA-UD3 motherboard, capable of supporting 2 PCIe GPUs @ x16, or 4 @ x8.
Going down to 1 GPU I was getting 30-40 FPS, even when overclocking it to 1120/1500 (Near 7970 performance levels). The CPU wasn't at full load. So looks like a dual-GPU setup will get you more in this game.