I use add2psu, which basically connects the ATX connector of the second psu with a molex of the first. The psu I use are https://www.evga.com/products/product.aspx?pn=210-GQ-0750-V1 They do have a molex cable that can be plugged into a perif1 port.
Huh. I have the G2 750w and they don't come with any molex. And they were $30 more expensive. Dang