I bought 2 YN 560 II to be fired by my 600ex rt's in ETTL2 Mode, they work a charm . Obviously they have to be set manually. I.e. They augment the 2 600's.
It needs a bit of getting used to how to meter them effectively. But they work fine. Basically if outside I set the 600s in Ettl mode, fire off a tester. And then I use the YNs to fill any areas that are too dark. Use my meter to match the F no selected by the Ettl system.
I am also using them to augment my Bowens studio lights works perfectly, all manual of course. But at £42 each they are a real bargain.
Canon EOS 5DS R, Canon EF 70-200 F2.8 L Mk II IS USM, Canon EF 70-300 F4-5.6 L IS USM, EF 40mm F2.8 STM , RC6 Remote. Canon STE-3 Radio Flash Controller, Canon 600 EX RT x4 , YN 560 MkII x2 ; Bowens GM500PRO x4 , Bowens Remote Control. Bowens Pulsar TX, RX Radio Transmitter and Reciever Cards. Bowens Constant 530 Streamlights 600w x 4 Sold EOS 5D Mk III, 7D, EF 50mm F1.8, 430 EX Mk II, Bowens GM500Rs x4