Well you can do that just using the 818 in its zone. Still no reason to connect it to the 861 as the 861 zone will still use the ID40 card.

MD600(2x2TB); iPad Pro
LR. 818v3, 8000SE
HT. 861v8+ID41,HD621; BW 2x802D,HTM1D,4x804S;SVS sub; Mac 5 MC501s; RGP 600Pro, JVC X950; Radiance Pro; Oppo 203
FR. G65, DSP3200-3300 DSW, HD621; Mac MC152; Oppo 103, PDP TCP65VT60, Radiance 2124; RGP1200C
4. G65, DSP3200-3300,HD621; OPPO 83, Radiance XE PDP
5. 818v3; Prime HP Amp; HD800