I have a similar setup here running without issues: Ryzen 5950x, 128GB RAM, 4090 + 3090 (previously dual 3090s), AX1600i PSU
I don't use C4D. I use Standalone and Modo, but can confirm Octanebench runs without problems too. Here's a couple of things to check while troubleshooting:
1. Make sure you have 1 PCIe power cable per 8-pin connection on the card.
eg. If your 2 cards have 3x8-pins each, then you'll need 6 separate PCIe cables.
2. If you are using any of the dual pigtail cables, don't connect the second pigtail.
Technically, connecting a pigtail cable to 2 card connections might work fine on this PSU, but avoid it for now just to be sure.
This PSU can be controlled by Corsair iCUE and has some settings to try/check:
3. Go to iCUE > ax1600i > device settings. Make sure each cable is set to 40A/40% (the default).
NOTE: I'm using iCUE v4.23.137 because I noticed a bug in newer builds which can set the PSU OCP amperage to the lowest setting (20A/20%).
4. You can also try running the PSU in Single Rail mode. iCUE > ax1600i > device settings > Single OCP. This should not be needed though.