Was never able to get any information out of AMD, so I bought a used FirePro S7150x2 and it works as anticipated, so the issue does appear to be the RX550 does not support mxgpu.
Was never able to get any information out of AMD, so I bought a used FirePro S7150x2 and it works as anticipated, so the issue does appear to be the RX550 does not support mxgpu.