What’s the safest way to power a second GPU from my 1000W PSU?

It may not be worth forcing this setup with adapters. A second GPU adds heat, power draw, and possible driver or application complications, so either use a low-power card that needs no auxiliary connector or move to a PSU with enough native connections for both GPUs.

A 12VHPWR-to-dual-8-pin PCIe adapter can provide two additional PCIe power plugs, but only use one specifically rated and wired for your exact PSU. Don’t assume that every modular PSU cable is interchangeable—PSU-side pinouts can differ. Ideally, get the cable directly from the PSU manufacturer or a reputable cable maker that explicitly lists this model and its pinout.

The simplest and safest option is choosing a secondary GPU that draws all its power from the motherboard slot, which is limited to roughly 75 W. Cards such as the RX 6400 or Intel Arc A380 generally avoid the need for another PSU connector, though you should still check the exact model and whether its performance features work well with your setup.

Before buying anything, check the exact power connectors on the 3090 and whether the 12VHPWR lead can be used with it through a manufacturer-approved cable. If you do add another PCIe cable, it must be made for the Pure Power 12M series; never use a modular cable from a different PSU. If there isn’t a documented cable arrangement you trust, replacing the PSU with a model offering more native PCIe outputs is the safer solution.