Do you have the SLI ribbon strip connector connected between both cards? Also is your power supply up to the task of 2 x 570s ie. is it a good brand with enough watts?
Yes, the two cards are connected with the ribbon; the ribbon is removed for the single-card setup. The power supply is a 1000 Watt Silverstone Strider Gold ST-1000G. I think this is a very good power supply. Also, the cpu is an Intel I7-2600K.