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How to run dual RTX4090 on ASUS Z790

I need some help with a build.
The problem happens when both GPUs (RTX4090’s) are plugged in at the same time.
When one is plugged in at a time, it works fine. Doesn’t matter which PCI slot. So I’ve eliminated the cards and motherboard slots as the issue.
Below are the full specs.

Any help or guidance would be greatly appreciated.

Case- Thermaltake The Tower
Motherboard- ASUS ROG Maximus Extreme Z790
CPU- Intel i9-13900K 13th Gen Raptor Lake
RAM- 128GB DDR5 Corsair Vengeance
Graphics- 2 x ASUS ROG Strix GeForce RTX 4090 OC Editions
Storage- Corsair M.2 NVMe Gen4- 2TB
Power Supply- Corsair AX1600i
Cooling:
Dual- EKWB Radiators
Thermaltake ring fans
14mm Rigid Tubing
2 Barrel reservoirs with pumps
CPU water block- Aqua computer
GPU water blocks- Bykski