[DSMan195276]

Keep in mind GPUs (and the rest of the computer) run on DC, not AC, so there is no phase by the time it comes to your computer. The PSU will step down the AC to the right voltage and then rectify it into DC, and they do that independently so whatever phase they started with shouldn't matter.

Something to keep in mind though is that (at least with consumer-grade PSUs) it is not safe to simply tie the outputs together, even if both PSUs produce 5V, 12V, 3.3V, etc. The voltages will be slightly different and connecting them together will cause current to flow back into one of the PSUs.

You can still use this setup though, the key is that the GPUs do not (or should not) connect the motherboard voltage provided via the card slot to the voltage provided via the power connector. This detail allows you to safely power the motherboard from one PSU and power the GPU from another one, you just have to be careful not to mix connectors on the same card between different PSUs (if it has multiple). Additionally the motherboard should be entirely powered from a single PSU.