You can connect the SATA to the primary PSU. Alternatively you can use your own molex connection.
The secondary PSU gets connected to the 24-pin, and this way when the primary starts up, it also triggers the secondary.

Easier to just loop pins 3 & 4 in my opinion, if you have a spare molex or sata power cable, just chop that about 3cm long and strip 3-4mm off the each end. As long as the PSU has power from the wall, it'll remain on.