I don't think that the analog outputs from the 818 will be a problem. What might be a problem is who will be the controller? In two-channel mode the 818 will be the processor and in 5.2 the 568 will be the processor, right? Can the 568 be controller in two-channel mode?

861v8+ID41, 3x DSP5200SE (RLC), analogue rears, 2x analogue subs, Oppo BDP-81, 568, Linn Sondek LP12, Stax SRM-006tS, M80, MS200, AppleTV4k.