The simplest way is to use a network failover switch to switch between the SpeakerLink outputs of the 818 and 861 then on to the SpeakerLink speakers. No special configurations involved. Additionally run a digital coax from the 818 to the 861 for Trifield.

Alternatively, you can go 861 SpeakerLink to 818 then from the 818 to the SpeakerLink speakers which uses the 818 in a "pass-through" mode. This requires each processor to have a unique configuration that complements the other in terms of source configuration. There are some comms issues to overcome that can be unique to each system. I have done this many times.

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#1 Trinnov Altitude 32-8-16, 271, 8000XE, 7200SEHC, 7200.2, Seaton F18, JL Fathom F113, Revel w990's, Kef R7, ATI 528 Amp, JVC RS4100K, SI screen. Roon Nucleus+, Eno AG, Eno M link, 218.
#2 Mark Levinson No5105, Ayre VX8 and CX8, Vandersteen Treo CT, Preamp varies.
#3 Meridian Ellipse