Having trouble finding a compatible PCIe power connector for the dual GPU build

My new PC build freezes frequently when under heavy load, mainly when rendering models in Redshift which uses both GPUs (full version below). When I disable my second GPU, the lockups no longer occur. Especially considering the power consumption of the 3090 (and this PC has two), it leads me to believe that the problem is related to power consumption, but I could be wrong. The motherboard instruction manual recommends for dual GPU builds to connect an additional PCIe power connector cable from the power supply to the motherboard, but I’m having trouble finding the correct cable. Any help is greatly appreciated.

Photo of motherboard instruction manual: https://postimg.cc/WhtfSmhy

PC version:
CPU: AMD Threadripper 3990X 2.9GHz 64 Core Processor
CPU cooler: NZXT Kraken Z73 73.11 CFM Liquid CPU Cooler
Motherboard : Gigabyte TRX40 AORUS XTREME XL ATX sTRX4 Motherboard
RAM: G.Skill Trident Z RGB 128GB (4 x 32GB) DDR4-4000 CL18 Memory
Storage: Samsung 980 Pro 2TB M.2-2280 NVME SSD
GPUs: Asus GeForce RTX 3090 24GB STRIX GAMING video card x2
Case:
Corsair 780T ATX Full Tower Case
Power source: Corsair AXi 1600W 80+ Titanium Certified Fully Modular ATX Power Supply

Alan A. Seibert