Running two PSUs to a GPU is generally fine. You don't completely parallel them, you just use the 12V outputs of the secondary to power the PCIe power plugs on the second GPU. You can buy adapters that turn on the second PSU based on the status of the first one, but it doesn't cause an issue* to power up the PCIe plugs before the switching on your computer.
*Just make sure you remember to shut off the other PSU if you take the card out with the cables attached and set it down somewhere metallish.